diff --git a/end_to_end_tests/golden-record-custom/custom_e2e/api/tests/defaults_tests_defaults_post.py b/end_to_end_tests/golden-record-custom/custom_e2e/api/tests/defaults_tests_defaults_post.py
index 574d5018b..fde6cb49b 100644
--- a/end_to_end_tests/golden-record-custom/custom_e2e/api/tests/defaults_tests_defaults_post.py
+++ b/end_to_end_tests/golden-record-custom/custom_e2e/api/tests/defaults_tests_defaults_post.py
@@ -40,28 +40,28 @@ def _build_response(*, response: httpx.Response) -> Response[Union[None, HTTPVal
 def httpx_request(
     *,
     client: Client,
-    string_prop: Union[Unset, str] = "the default string",
-    datetime_prop: Union[Unset, datetime.datetime] = isoparse("1010-10-10T00:00:00"),
-    date_prop: Union[Unset, datetime.date] = isoparse("1010-10-10").date(),
-    float_prop: Union[Unset, float] = 3.14,
-    int_prop: Union[Unset, int] = 7,
-    boolean_prop: Union[Unset, bool] = False,
-    list_prop: Union[Unset, List[AnEnum]] = UNSET,
-    union_prop: Union[Unset, float, str] = "not a float",
-    union_prop_with_ref: Union[Unset, float, AnEnum] = 0.6,
-    enum_prop: Union[Unset, AnEnum] = UNSET,
+    string_prop: Union[Unset, None, str] = "the default string",
+    datetime_prop: Union[Unset, None, datetime.datetime] = isoparse("1010-10-10T00:00:00"),
+    date_prop: Union[Unset, None, datetime.date] = isoparse("1010-10-10").date(),
+    float_prop: Union[Unset, None, float] = 3.14,
+    int_prop: Union[Unset, None, int] = 7,
+    boolean_prop: Union[Unset, None, bool] = False,
+    list_prop: Union[Unset, None, List[AnEnum]] = None,
+    union_prop: Union[Unset, None, float, str] = "not a float",
+    union_prop_with_ref: Union[Unset, None, float, AnEnum] = 0.6,
+    enum_prop: Union[Unset, None, AnEnum] = None,
 ) -> Response[Union[None, HTTPValidationError]]:
 
     json_datetime_prop: Union[Unset, str] = UNSET
-    if not isinstance(datetime_prop, Unset):
+    if not isinstance(datetime_prop, Unset) and datetime_prop is not None:
         json_datetime_prop = datetime_prop.isoformat()
 
     json_date_prop: Union[Unset, str] = UNSET
-    if not isinstance(date_prop, Unset):
+    if not isinstance(date_prop, Unset) and date_prop is not None:
         json_date_prop = date_prop.isoformat()
 
     json_list_prop: Union[Unset, List[Any]] = UNSET
-    if not isinstance(list_prop, Unset):
+    if not isinstance(list_prop, Unset) and list_prop is not None:
         json_list_prop = []
         for list_prop_item_data in list_prop:
             list_prop_item = list_prop_item_data.value
@@ -69,13 +69,13 @@ def httpx_request(
             json_list_prop.append(list_prop_item)
 
     json_union_prop: Union[Unset, float, str]
-    if isinstance(union_prop, Unset):
+    if isinstance(union_prop, Unset) or union_prop is None:
         json_union_prop = UNSET
     else:
         json_union_prop = union_prop
 
     json_union_prop_with_ref: Union[Unset, float, AnEnum]
-    if isinstance(union_prop_with_ref, Unset):
+    if isinstance(union_prop_with_ref, Unset) or union_prop_with_ref is None:
         json_union_prop_with_ref = UNSET
     elif isinstance(union_prop_with_ref, AnEnum):
         json_union_prop_with_ref = UNSET
@@ -86,29 +86,29 @@ def httpx_request(
         json_union_prop_with_ref = union_prop_with_ref
 
     json_enum_prop: Union[Unset, AnEnum] = UNSET
-    if not isinstance(enum_prop, Unset):
+    if not isinstance(enum_prop, Unset) and enum_prop is not None:
         json_enum_prop = enum_prop
 
     params: Dict[str, Any] = {}
-    if string_prop is not UNSET:
+    if string_prop is not UNSET and string_prop is not None:
         params["string_prop"] = string_prop
-    if datetime_prop is not UNSET:
+    if datetime_prop is not UNSET and datetime_prop is not None:
         params["datetime_prop"] = json_datetime_prop
-    if date_prop is not UNSET:
+    if date_prop is not UNSET and date_prop is not None:
         params["date_prop"] = json_date_prop
-    if float_prop is not UNSET:
+    if float_prop is not UNSET and float_prop is not None:
         params["float_prop"] = float_prop
-    if int_prop is not UNSET:
+    if int_prop is not UNSET and int_prop is not None:
         params["int_prop"] = int_prop
-    if boolean_prop is not UNSET:
+    if boolean_prop is not UNSET and boolean_prop is not None:
         params["boolean_prop"] = boolean_prop
-    if list_prop is not UNSET:
+    if list_prop is not UNSET and list_prop is not None:
         params["list_prop"] = json_list_prop
-    if union_prop is not UNSET:
+    if union_prop is not UNSET and union_prop is not None:
         params["union_prop"] = json_union_prop
-    if union_prop_with_ref is not UNSET:
+    if union_prop_with_ref is not UNSET and union_prop_with_ref is not None:
         params["union_prop_with_ref"] = json_union_prop_with_ref
-    if enum_prop is not UNSET:
+    if enum_prop is not UNSET and enum_prop is not None:
         params["enum_prop"] = json_enum_prop
 
     response = client.request(
diff --git a/end_to_end_tests/golden-record-custom/custom_e2e/api/tests/optional_value_tests_optional_query_param.py b/end_to_end_tests/golden-record-custom/custom_e2e/api/tests/optional_value_tests_optional_query_param.py
index bff43cc10..7b821fca6 100644
--- a/end_to_end_tests/golden-record-custom/custom_e2e/api/tests/optional_value_tests_optional_query_param.py
+++ b/end_to_end_tests/golden-record-custom/custom_e2e/api/tests/optional_value_tests_optional_query_param.py
@@ -36,15 +36,15 @@ def _build_response(*, response: httpx.Response) -> Response[Union[None, HTTPVal
 def httpx_request(
     *,
     client: Client,
-    query_param: Union[Unset, List[str]] = UNSET,
+    query_param: Union[Unset, None, List[str]] = None,
 ) -> Response[Union[None, HTTPValidationError]]:
 
     json_query_param: Union[Unset, List[Any]] = UNSET
-    if not isinstance(query_param, Unset):
+    if not isinstance(query_param, Unset) and query_param is not None:
         json_query_param = query_param
 
     params: Dict[str, Any] = {}
-    if query_param is not UNSET:
+    if query_param is not UNSET and query_param is not None:
         params["query_param"] = json_query_param
 
     response = client.request(
diff --git a/end_to_end_tests/golden-record-custom/custom_e2e/models/model_with_primitive_additional_properties.py b/end_to_end_tests/golden-record-custom/custom_e2e/models/model_with_primitive_additional_properties.py
index 47d65d90b..feb30878d 100644
--- a/end_to_end_tests/golden-record-custom/custom_e2e/models/model_with_primitive_additional_properties.py
+++ b/end_to_end_tests/golden-record-custom/custom_e2e/models/model_with_primitive_additional_properties.py
@@ -12,7 +12,7 @@
 class ModelWithPrimitiveAdditionalProperties:
     """  """
 
-    a_date_holder: Union[ModelWithPrimitiveAdditionalPropertiesADateHolder, Unset] = UNSET
+    a_date_holder: Union[Unset, ModelWithPrimitiveAdditionalPropertiesADateHolder] = UNSET
     additional_properties: Dict[str, str] = attr.ib(init=False, factory=dict)
 
     def to_dict(self) -> Dict[str, Any]:
@@ -31,7 +31,7 @@ def to_dict(self) -> Dict[str, Any]:
     @staticmethod
     def from_dict(src_dict: Dict[str, Any]) -> "ModelWithPrimitiveAdditionalProperties":
         d = src_dict.copy()
-        a_date_holder: Union[ModelWithPrimitiveAdditionalPropertiesADateHolder, Unset] = UNSET
+        a_date_holder: Union[Unset, ModelWithPrimitiveAdditionalPropertiesADateHolder] = UNSET
         _a_date_holder = d.pop("a_date_holder", UNSET)
         if _a_date_holder is not None and not isinstance(_a_date_holder, Unset):
             a_date_holder = ModelWithPrimitiveAdditionalPropertiesADateHolder.from_dict(
diff --git a/end_to_end_tests/golden-record/my_test_api_client/api/tests/defaults_tests_defaults_post.py b/end_to_end_tests/golden-record/my_test_api_client/api/tests/defaults_tests_defaults_post.py
index 9242cddaa..77ee6cce1 100644
--- a/end_to_end_tests/golden-record/my_test_api_client/api/tests/defaults_tests_defaults_post.py
+++ b/end_to_end_tests/golden-record/my_test_api_client/api/tests/defaults_tests_defaults_post.py
@@ -13,31 +13,31 @@
 def _get_kwargs(
     *,
     client: Client,
-    string_prop: Union[Unset, str] = "the default string",
-    datetime_prop: Union[Unset, datetime.datetime] = isoparse("1010-10-10T00:00:00"),
-    date_prop: Union[Unset, datetime.date] = isoparse("1010-10-10").date(),
-    float_prop: Union[Unset, float] = 3.14,
-    int_prop: Union[Unset, int] = 7,
-    boolean_prop: Union[Unset, bool] = False,
-    list_prop: Union[Unset, List[AnEnum]] = UNSET,
-    union_prop: Union[Unset, float, str] = "not a float",
-    union_prop_with_ref: Union[Unset, float, AnEnum] = 0.6,
-    enum_prop: Union[Unset, AnEnum] = UNSET,
+    string_prop: Union[Unset, None, str] = "the default string",
+    datetime_prop: Union[Unset, None, datetime.datetime] = isoparse("1010-10-10T00:00:00"),
+    date_prop: Union[Unset, None, datetime.date] = isoparse("1010-10-10").date(),
+    float_prop: Union[Unset, None, float] = 3.14,
+    int_prop: Union[Unset, None, int] = 7,
+    boolean_prop: Union[Unset, None, bool] = False,
+    list_prop: Union[Unset, None, List[AnEnum]] = None,
+    union_prop: Union[Unset, None, float, str] = "not a float",
+    union_prop_with_ref: Union[Unset, None, float, AnEnum] = 0.6,
+    enum_prop: Union[Unset, None, AnEnum] = None,
 ) -> Dict[str, Any]:
     url = "{}/tests/defaults".format(client.base_url)
 
     headers: Dict[str, Any] = client.get_headers()
 
     json_datetime_prop: Union[Unset, str] = UNSET
-    if not isinstance(datetime_prop, Unset):
+    if not isinstance(datetime_prop, Unset) and datetime_prop is not None:
         json_datetime_prop = datetime_prop.isoformat()
 
     json_date_prop: Union[Unset, str] = UNSET
-    if not isinstance(date_prop, Unset):
+    if not isinstance(date_prop, Unset) and date_prop is not None:
         json_date_prop = date_prop.isoformat()
 
     json_list_prop: Union[Unset, List[Any]] = UNSET
-    if not isinstance(list_prop, Unset):
+    if not isinstance(list_prop, Unset) and list_prop is not None:
         json_list_prop = []
         for list_prop_item_data in list_prop:
             list_prop_item = list_prop_item_data.value
@@ -45,13 +45,13 @@ def _get_kwargs(
             json_list_prop.append(list_prop_item)
 
     json_union_prop: Union[Unset, float, str]
-    if isinstance(union_prop, Unset):
+    if isinstance(union_prop, Unset) or union_prop is None:
         json_union_prop = UNSET
     else:
         json_union_prop = union_prop
 
     json_union_prop_with_ref: Union[Unset, float, AnEnum]
-    if isinstance(union_prop_with_ref, Unset):
+    if isinstance(union_prop_with_ref, Unset) or union_prop_with_ref is None:
         json_union_prop_with_ref = UNSET
     elif isinstance(union_prop_with_ref, AnEnum):
         json_union_prop_with_ref = UNSET
@@ -62,29 +62,29 @@ def _get_kwargs(
         json_union_prop_with_ref = union_prop_with_ref
 
     json_enum_prop: Union[Unset, AnEnum] = UNSET
-    if not isinstance(enum_prop, Unset):
+    if not isinstance(enum_prop, Unset) and enum_prop is not None:
         json_enum_prop = enum_prop
 
     params: Dict[str, Any] = {}
-    if string_prop is not UNSET:
+    if string_prop is not UNSET and string_prop is not None:
         params["string_prop"] = string_prop
-    if datetime_prop is not UNSET:
+    if datetime_prop is not UNSET and datetime_prop is not None:
         params["datetime_prop"] = json_datetime_prop
-    if date_prop is not UNSET:
+    if date_prop is not UNSET and date_prop is not None:
         params["date_prop"] = json_date_prop
-    if float_prop is not UNSET:
+    if float_prop is not UNSET and float_prop is not None:
         params["float_prop"] = float_prop
-    if int_prop is not UNSET:
+    if int_prop is not UNSET and int_prop is not None:
         params["int_prop"] = int_prop
-    if boolean_prop is not UNSET:
+    if boolean_prop is not UNSET and boolean_prop is not None:
         params["boolean_prop"] = boolean_prop
-    if list_prop is not UNSET:
+    if list_prop is not UNSET and list_prop is not None:
         params["list_prop"] = json_list_prop
-    if union_prop is not UNSET:
+    if union_prop is not UNSET and union_prop is not None:
         params["union_prop"] = json_union_prop
-    if union_prop_with_ref is not UNSET:
+    if union_prop_with_ref is not UNSET and union_prop_with_ref is not None:
         params["union_prop_with_ref"] = json_union_prop_with_ref
-    if enum_prop is not UNSET:
+    if enum_prop is not UNSET and enum_prop is not None:
         params["enum_prop"] = json_enum_prop
 
     return {
@@ -120,16 +120,16 @@ def _build_response(*, response: httpx.Response) -> Response[Union[None, HTTPVal
 def sync_detailed(
     *,
     client: Client,
-    string_prop: Union[Unset, str] = "the default string",
-    datetime_prop: Union[Unset, datetime.datetime] = isoparse("1010-10-10T00:00:00"),
-    date_prop: Union[Unset, datetime.date] = isoparse("1010-10-10").date(),
-    float_prop: Union[Unset, float] = 3.14,
-    int_prop: Union[Unset, int] = 7,
-    boolean_prop: Union[Unset, bool] = False,
-    list_prop: Union[Unset, List[AnEnum]] = UNSET,
-    union_prop: Union[Unset, float, str] = "not a float",
-    union_prop_with_ref: Union[Unset, float, AnEnum] = 0.6,
-    enum_prop: Union[Unset, AnEnum] = UNSET,
+    string_prop: Union[Unset, None, str] = "the default string",
+    datetime_prop: Union[Unset, None, datetime.datetime] = isoparse("1010-10-10T00:00:00"),
+    date_prop: Union[Unset, None, datetime.date] = isoparse("1010-10-10").date(),
+    float_prop: Union[Unset, None, float] = 3.14,
+    int_prop: Union[Unset, None, int] = 7,
+    boolean_prop: Union[Unset, None, bool] = False,
+    list_prop: Union[Unset, None, List[AnEnum]] = None,
+    union_prop: Union[Unset, None, float, str] = "not a float",
+    union_prop_with_ref: Union[Unset, None, float, AnEnum] = 0.6,
+    enum_prop: Union[Unset, None, AnEnum] = None,
 ) -> Response[Union[None, HTTPValidationError]]:
     kwargs = _get_kwargs(
         client=client,
@@ -155,16 +155,16 @@ def sync_detailed(
 def sync(
     *,
     client: Client,
-    string_prop: Union[Unset, str] = "the default string",
-    datetime_prop: Union[Unset, datetime.datetime] = isoparse("1010-10-10T00:00:00"),
-    date_prop: Union[Unset, datetime.date] = isoparse("1010-10-10").date(),
-    float_prop: Union[Unset, float] = 3.14,
-    int_prop: Union[Unset, int] = 7,
-    boolean_prop: Union[Unset, bool] = False,
-    list_prop: Union[Unset, List[AnEnum]] = UNSET,
-    union_prop: Union[Unset, float, str] = "not a float",
-    union_prop_with_ref: Union[Unset, float, AnEnum] = 0.6,
-    enum_prop: Union[Unset, AnEnum] = UNSET,
+    string_prop: Union[Unset, None, str] = "the default string",
+    datetime_prop: Union[Unset, None, datetime.datetime] = isoparse("1010-10-10T00:00:00"),
+    date_prop: Union[Unset, None, datetime.date] = isoparse("1010-10-10").date(),
+    float_prop: Union[Unset, None, float] = 3.14,
+    int_prop: Union[Unset, None, int] = 7,
+    boolean_prop: Union[Unset, None, bool] = False,
+    list_prop: Union[Unset, None, List[AnEnum]] = None,
+    union_prop: Union[Unset, None, float, str] = "not a float",
+    union_prop_with_ref: Union[Unset, None, float, AnEnum] = 0.6,
+    enum_prop: Union[Unset, None, AnEnum] = None,
 ) -> Optional[Union[None, HTTPValidationError]]:
     """  """
 
@@ -186,16 +186,16 @@ def sync(
 async def asyncio_detailed(
     *,
     client: Client,
-    string_prop: Union[Unset, str] = "the default string",
-    datetime_prop: Union[Unset, datetime.datetime] = isoparse("1010-10-10T00:00:00"),
-    date_prop: Union[Unset, datetime.date] = isoparse("1010-10-10").date(),
-    float_prop: Union[Unset, float] = 3.14,
-    int_prop: Union[Unset, int] = 7,
-    boolean_prop: Union[Unset, bool] = False,
-    list_prop: Union[Unset, List[AnEnum]] = UNSET,
-    union_prop: Union[Unset, float, str] = "not a float",
-    union_prop_with_ref: Union[Unset, float, AnEnum] = 0.6,
-    enum_prop: Union[Unset, AnEnum] = UNSET,
+    string_prop: Union[Unset, None, str] = "the default string",
+    datetime_prop: Union[Unset, None, datetime.datetime] = isoparse("1010-10-10T00:00:00"),
+    date_prop: Union[Unset, None, datetime.date] = isoparse("1010-10-10").date(),
+    float_prop: Union[Unset, None, float] = 3.14,
+    int_prop: Union[Unset, None, int] = 7,
+    boolean_prop: Union[Unset, None, bool] = False,
+    list_prop: Union[Unset, None, List[AnEnum]] = None,
+    union_prop: Union[Unset, None, float, str] = "not a float",
+    union_prop_with_ref: Union[Unset, None, float, AnEnum] = 0.6,
+    enum_prop: Union[Unset, None, AnEnum] = None,
 ) -> Response[Union[None, HTTPValidationError]]:
     kwargs = _get_kwargs(
         client=client,
@@ -220,16 +220,16 @@ async def asyncio_detailed(
 async def asyncio(
     *,
     client: Client,
-    string_prop: Union[Unset, str] = "the default string",
-    datetime_prop: Union[Unset, datetime.datetime] = isoparse("1010-10-10T00:00:00"),
-    date_prop: Union[Unset, datetime.date] = isoparse("1010-10-10").date(),
-    float_prop: Union[Unset, float] = 3.14,
-    int_prop: Union[Unset, int] = 7,
-    boolean_prop: Union[Unset, bool] = False,
-    list_prop: Union[Unset, List[AnEnum]] = UNSET,
-    union_prop: Union[Unset, float, str] = "not a float",
-    union_prop_with_ref: Union[Unset, float, AnEnum] = 0.6,
-    enum_prop: Union[Unset, AnEnum] = UNSET,
+    string_prop: Union[Unset, None, str] = "the default string",
+    datetime_prop: Union[Unset, None, datetime.datetime] = isoparse("1010-10-10T00:00:00"),
+    date_prop: Union[Unset, None, datetime.date] = isoparse("1010-10-10").date(),
+    float_prop: Union[Unset, None, float] = 3.14,
+    int_prop: Union[Unset, None, int] = 7,
+    boolean_prop: Union[Unset, None, bool] = False,
+    list_prop: Union[Unset, None, List[AnEnum]] = None,
+    union_prop: Union[Unset, None, float, str] = "not a float",
+    union_prop_with_ref: Union[Unset, None, float, AnEnum] = 0.6,
+    enum_prop: Union[Unset, None, AnEnum] = None,
 ) -> Optional[Union[None, HTTPValidationError]]:
     """  """
 
diff --git a/end_to_end_tests/golden-record/my_test_api_client/api/tests/optional_value_tests_optional_query_param.py b/end_to_end_tests/golden-record/my_test_api_client/api/tests/optional_value_tests_optional_query_param.py
index 751f48e03..8058acb43 100644
--- a/end_to_end_tests/golden-record/my_test_api_client/api/tests/optional_value_tests_optional_query_param.py
+++ b/end_to_end_tests/golden-record/my_test_api_client/api/tests/optional_value_tests_optional_query_param.py
@@ -10,18 +10,18 @@
 def _get_kwargs(
     *,
     client: Client,
-    query_param: Union[Unset, List[str]] = UNSET,
+    query_param: Union[Unset, None, List[str]] = None,
 ) -> Dict[str, Any]:
     url = "{}/tests/optional_query_param/".format(client.base_url)
 
     headers: Dict[str, Any] = client.get_headers()
 
     json_query_param: Union[Unset, List[Any]] = UNSET
-    if not isinstance(query_param, Unset):
+    if not isinstance(query_param, Unset) and query_param is not None:
         json_query_param = query_param
 
     params: Dict[str, Any] = {}
-    if query_param is not UNSET:
+    if query_param is not UNSET and query_param is not None:
         params["query_param"] = json_query_param
 
     return {
@@ -57,7 +57,7 @@ def _build_response(*, response: httpx.Response) -> Response[Union[None, HTTPVal
 def sync_detailed(
     *,
     client: Client,
-    query_param: Union[Unset, List[str]] = UNSET,
+    query_param: Union[Unset, None, List[str]] = None,
 ) -> Response[Union[None, HTTPValidationError]]:
     kwargs = _get_kwargs(
         client=client,
@@ -74,7 +74,7 @@ def sync_detailed(
 def sync(
     *,
     client: Client,
-    query_param: Union[Unset, List[str]] = UNSET,
+    query_param: Union[Unset, None, List[str]] = None,
 ) -> Optional[Union[None, HTTPValidationError]]:
     """ Test optional query parameters """
 
@@ -87,7 +87,7 @@ def sync(
 async def asyncio_detailed(
     *,
     client: Client,
-    query_param: Union[Unset, List[str]] = UNSET,
+    query_param: Union[Unset, None, List[str]] = None,
 ) -> Response[Union[None, HTTPValidationError]]:
     kwargs = _get_kwargs(
         client=client,
@@ -103,7 +103,7 @@ async def asyncio_detailed(
 async def asyncio(
     *,
     client: Client,
-    query_param: Union[Unset, List[str]] = UNSET,
+    query_param: Union[Unset, None, List[str]] = None,
 ) -> Optional[Union[None, HTTPValidationError]]:
     """ Test optional query parameters """
 
diff --git a/end_to_end_tests/golden-record/my_test_api_client/models/model_with_primitive_additional_properties.py b/end_to_end_tests/golden-record/my_test_api_client/models/model_with_primitive_additional_properties.py
index 47d65d90b..feb30878d 100644
--- a/end_to_end_tests/golden-record/my_test_api_client/models/model_with_primitive_additional_properties.py
+++ b/end_to_end_tests/golden-record/my_test_api_client/models/model_with_primitive_additional_properties.py
@@ -12,7 +12,7 @@
 class ModelWithPrimitiveAdditionalProperties:
     """  """
 
-    a_date_holder: Union[ModelWithPrimitiveAdditionalPropertiesADateHolder, Unset] = UNSET
+    a_date_holder: Union[Unset, ModelWithPrimitiveAdditionalPropertiesADateHolder] = UNSET
     additional_properties: Dict[str, str] = attr.ib(init=False, factory=dict)
 
     def to_dict(self) -> Dict[str, Any]:
@@ -31,7 +31,7 @@ def to_dict(self) -> Dict[str, Any]:
     @staticmethod
     def from_dict(src_dict: Dict[str, Any]) -> "ModelWithPrimitiveAdditionalProperties":
         d = src_dict.copy()
-        a_date_holder: Union[ModelWithPrimitiveAdditionalPropertiesADateHolder, Unset] = UNSET
+        a_date_holder: Union[Unset, ModelWithPrimitiveAdditionalPropertiesADateHolder] = UNSET
         _a_date_holder = d.pop("a_date_holder", UNSET)
         if _a_date_holder is not None and not isinstance(_a_date_holder, Unset):
             a_date_holder = ModelWithPrimitiveAdditionalPropertiesADateHolder.from_dict(
diff --git a/openapi_python_client/parser/properties/__init__.py b/openapi_python_client/parser/properties/__init__.py
index 487f5b464..4669769c8 100644
--- a/openapi_python_client/parser/properties/__init__.py
+++ b/openapi_python_client/parser/properties/__init__.py
@@ -124,16 +124,8 @@ class ListProperty(Property, Generic[InnerProp]):
     inner_property: InnerProp
     template: ClassVar[str] = "list_property.pyi"
 
-    def get_type_string(self, no_optional: bool = False) -> str:
-        """ Get a string representation of type that should be used when declaring this property """
-        type_string = f"List[{self.inner_property.get_type_string()}]"
-        if no_optional:
-            return type_string
-        if self.nullable:
-            type_string = f"Optional[{type_string}]"
-        if not self.required:
-            type_string = f"Union[Unset, {type_string}]"
-        return type_string
+    def get_base_type_string(self) -> str:
+        return f"List[{self.inner_property.get_type_string()}]"
 
     def get_instance_type_string(self) -> str:
         """Get a string representation of runtime type that should be used for `isinstance` checks"""
@@ -167,17 +159,26 @@ def __attrs_post_init__(self) -> None:
             self, "has_properties_without_templates", any(prop.template is None for prop in self.inner_properties)
         )
 
-    def get_type_string(self, no_optional: bool = False) -> str:
-        """ Get a string representation of type that should be used when declaring this property """
+    def _get_inner_prop_string(self) -> List[str]:
         inner_types = [p.get_type_string(no_optional=True) for p in self.inner_properties]
-        inner_prop_string = ", ".join(inner_types)
-        type_string = f"Union[{inner_prop_string}]"
+        return ", ".join(inner_types)
+    
+    def get_base_type_string(self) -> str:
+        return f"Union[{self._get_inner_prop_string()}]"
+
+    def get_type_string(self, no_optional: bool = False, query_parameter: bool = False) -> str:
+        """ Get a string representation of type that should be used when declaring this property """
+        type_string = self.get_base_type_string()
         if no_optional:
             return type_string
-        if not self.required:
-            type_string = f"Union[Unset, {inner_prop_string}]"
         if self.nullable:
             type_string = f"Optional[{type_string}]"
+        if not self.required:
+            if query_parameter:
+                # For query parameters, None has the same meaning as Unset
+                type_string = f"Union[Unset, None, {self._get_inner_prop_string()}]"
+            else:
+                type_string = f"Union[Unset, {self._get_inner_prop_string()}]"
         return type_string
 
     def get_imports(self, *, prefix: str) -> Set[str]:
diff --git a/openapi_python_client/parser/properties/enum_property.py b/openapi_python_client/parser/properties/enum_property.py
index 1217f23ee..08fce294d 100644
--- a/openapi_python_client/parser/properties/enum_property.py
+++ b/openapi_python_client/parser/properties/enum_property.py
@@ -22,17 +22,8 @@ class EnumProperty(Property):
 
     template: ClassVar[str] = "enum_property.pyi"
 
-    def get_type_string(self, no_optional: bool = False) -> str:
-        """ Get a string representation of type that should be used when declaring this property """
-
-        type_string = self.reference.class_name
-        if no_optional:
-            return type_string
-        if self.nullable:
-            type_string = f"Optional[{type_string}]"
-        if not self.required:
-            type_string = f"Union[Unset, {type_string}]"
-        return type_string
+    def get_base_type_string(self) -> str:
+        return self.reference.class_name
 
     def get_imports(self, *, prefix: str) -> Set[str]:
         """
diff --git a/openapi_python_client/parser/properties/model_property.py b/openapi_python_client/parser/properties/model_property.py
index 40d1f930b..7ebc804b2 100644
--- a/openapi_python_client/parser/properties/model_property.py
+++ b/openapi_python_client/parser/properties/model_property.py
@@ -71,16 +71,8 @@ def resolve_references(
 
         return schemas
 
-    def get_type_string(self, no_optional: bool = False) -> str:
-        """ Get a string representation of type that should be used when declaring this property """
-        type_string = self.reference.class_name
-        if no_optional:
-            return type_string
-        if self.nullable:
-            type_string = f"Optional[{type_string}]"
-        if not self.required:
-            type_string = f"Union[{type_string}, Unset]"
-        return type_string
+    def get_base_type_string(self) -> str:
+        return self.reference.class_name
 
     def get_imports(self, *, prefix: str) -> Set[str]:
         """
diff --git a/openapi_python_client/parser/properties/property.py b/openapi_python_client/parser/properties/property.py
index 0b7047551..a75fc481f 100644
--- a/openapi_python_client/parser/properties/property.py
+++ b/openapi_python_client/parser/properties/property.py
@@ -31,21 +31,29 @@ class Property:
 
     def __attrs_post_init__(self) -> None:
         object.__setattr__(self, "python_name", utils.to_valid_python_identifier(utils.snake_case(self.name)))
+    
+    def get_base_type_string(self) -> str:
+        return self._type_string
 
-    def get_type_string(self, no_optional: bool = False) -> str:
+    def get_type_string(self, no_optional: bool = False, query_parameter: bool = False) -> str:
         """
         Get a string representation of type that should be used when declaring this property
 
         Args:
             no_optional: Do not include Optional or Unset even if the value is optional (needed for isinstance checks)
+            query_parameter: True if the property's type is being used for a query parameter
         """
-        type_string = self._type_string
+        type_string = self.get_base_type_string()
         if no_optional:
-            return self._type_string
+            return type_string
         if self.nullable:
             type_string = f"Optional[{type_string}]"
         if not self.required:
-            type_string = f"Union[Unset, {type_string}]"
+            if query_parameter:
+                # For query parameters, None has the same meaning as Unset
+                type_string = f"Union[Unset, None, {type_string}]"
+            else:
+                type_string = f"Union[Unset, {type_string}]"
         return type_string
 
     def get_instance_type_string(self) -> str:
@@ -69,17 +77,25 @@ def get_imports(self, *, prefix: str) -> Set[str]:
             imports.add(f"from {prefix}types import UNSET, Unset")
         return imports
 
-    def to_string(self) -> str:
-        """ How this should be declared in a dataclass """
+    def to_string(self, query_parameter: bool = False) -> str:
+        """
+        How this should be declared in a dataclass
+        
+        Args:
+            query_parameter: True if the property's type is being used for a query parameter
+        """
         default: Optional[str]
         if self.default is not None:
             default = self.default
         elif not self.required:
-            default = "UNSET"
+            if query_parameter:
+                default = "None"
+            else:
+                default = "UNSET"
         else:
             default = None
 
         if default is not None:
-            return f"{self.python_name}: {self.get_type_string()} = {default}"
+            return f"{self.python_name}: {self.get_type_string(query_parameter=query_parameter)} = {default}"
         else:
-            return f"{self.python_name}: {self.get_type_string()}"
+            return f"{self.python_name}: {self.get_type_string(query_parameter=query_parameter)}"
diff --git a/openapi_python_client/templates/endpoint_macros.pyi b/openapi_python_client/templates/endpoint_macros.pyi
index 5819714d8..3e527b017 100644
--- a/openapi_python_client/templates/endpoint_macros.pyi
+++ b/openapi_python_client/templates/endpoint_macros.pyi
@@ -17,7 +17,7 @@ if {{ parameter.python_name }} is not UNSET:
         {% set destination = "json_" + property.python_name %}
         {% if property.template %}
             {% from "property_templates/" + property.template import transform %}
-{{ transform(property, property.python_name, destination) }}
+{{ transform(property, property.python_name, destination, query_parameter=True) }}
         {% endif %}
     {% endfor %}
 params: Dict[str, Any] = {
@@ -33,7 +33,7 @@ params: Dict[str, Any] = {
 }
     {% for property in endpoint.query_parameters %}
         {% if not property.required %}
-if {{ property.python_name }} is not UNSET:
+if {{ property.python_name }} is not UNSET and {{ property.python_name }} is not None:
             {% if property.template %}
     params["{{ property.name }}"] = {{ "json_" + property.python_name }}
             {% else %}
@@ -96,7 +96,7 @@ json_body: {{ endpoint.json_body.get_type_string() }},
 {% endif %}
 {# query parameters #}
 {% for parameter in endpoint.query_parameters %}
-{{ parameter.to_string() }},
+{{ parameter.to_string(query_parameter=True) }},
 {% endfor %}
 {% for parameter in endpoint.header_parameters %}
 {{ parameter.to_string() }},
diff --git a/openapi_python_client/templates/property_templates/date_property.pyi b/openapi_python_client/templates/property_templates/date_property.pyi
index a3a980c8f..18fbf7735 100644
--- a/openapi_python_client/templates/property_templates/date_property.pyi
+++ b/openapi_python_client/templates/property_templates/date_property.pyi
@@ -9,12 +9,12 @@ if _{{ property.python_name }} is not None:
 {% endif %}
 {% endmacro %}
 
-{% macro transform(property, source, destination, declare_type=True) %}
+{% macro transform(property, source, destination, declare_type=True, query_parameter=False) %}
 {% if property.required %}
 {{ destination }} = {{ source }}.isoformat() {% if property.nullable %}if {{ source }} else None {%endif%}
 {% else %}
 {{ destination }}{% if declare_type %}: Union[Unset, str]{% endif %} = UNSET
-if not isinstance({{ source }}, Unset):
+if not isinstance({{ source }}, Unset){%if query_parameter %} and {{ source }} is not None{% endif %}:
 {% if property.nullable %}
     {{ destination }} = {{ source }}.isoformat() if {{ source }} else None
 {% else %}
diff --git a/openapi_python_client/templates/property_templates/datetime_property.pyi b/openapi_python_client/templates/property_templates/datetime_property.pyi
index b8e1b8ff0..073db6822 100644
--- a/openapi_python_client/templates/property_templates/datetime_property.pyi
+++ b/openapi_python_client/templates/property_templates/datetime_property.pyi
@@ -14,7 +14,7 @@ if _{{ property.python_name }} is not None:
 {% endif %}
 {% endmacro %}
 
-{% macro transform(property, source, destination, declare_type=True) %}
+{% macro transform(property, source, destination, declare_type=True, query_parameter=False) %}
 {% if property.required %}
 {% if property.nullable %}
 {{ destination }} = {{ source }}.isoformat() if {{ source }} else None
@@ -23,7 +23,7 @@ if _{{ property.python_name }} is not None:
 {% endif %}
 {% else %}
 {{ destination }}{% if declare_type %}: Union[Unset, str]{% endif %} = UNSET
-if not isinstance({{ source }}, Unset):
+if not isinstance({{ source }}, Unset){%if query_parameter %} and {{ source }} is not None{% endif %}:
 {% if property.nullable %}
     {{ destination }} = {{ source }}.isoformat() if {{ source }} else None
 {% else %}
diff --git a/openapi_python_client/templates/property_templates/enum_property.pyi b/openapi_python_client/templates/property_templates/enum_property.pyi
index 4765a6fd5..a5f07a8e4 100644
--- a/openapi_python_client/templates/property_templates/enum_property.pyi
+++ b/openapi_python_client/templates/property_templates/enum_property.pyi
@@ -9,7 +9,7 @@ if _{{ property.python_name }} is not None:
 {% endif %}
 {% endmacro %}
 
-{% macro transform(property, source, destination, declare_type=True) %}
+{% macro transform(property, source, destination, declare_type=True, query_parameter=False) %}
 {% if property.required %}
 {% if property.nullable %}
 {{ destination }} = {{ source }}.value if {{ source }} else None
@@ -18,7 +18,7 @@ if _{{ property.python_name }} is not None:
 {% endif %}
 {% else %}
 {{ destination }}{% if declare_type %}: {{ property.get_type_string() }}{% endif %} = UNSET
-if not isinstance({{ source }}, Unset):
+if not isinstance({{ source }}, Unset){%if query_parameter %} and {{ source }} is not None{% endif %}:
 {% if property.nullable %}
     {{ destination }} = {{ source }} if {{ source }} else None
 {% else %}
diff --git a/openapi_python_client/templates/property_templates/file_property.pyi b/openapi_python_client/templates/property_templates/file_property.pyi
index ffa3c20d9..3833a93ea 100644
--- a/openapi_python_client/templates/property_templates/file_property.pyi
+++ b/openapi_python_client/templates/property_templates/file_property.pyi
@@ -4,7 +4,7 @@
 )
 {% endmacro %}
 
-{% macro transform(property, source, destination, declare_type=True) %}
+{% macro transform(property, source, destination, declare_type=True, query_parameter=False) %}
 {% if property.required %}
 {% if property.nullable %}
 {{ destination }} = {{ source }}.to_tuple() if {{ source }} else None
@@ -13,7 +13,7 @@
 {% endif %}
 {% else %}
 {{ destination }}{% if declare_type %}: {{ property.get_type_string() }}{% endif %} = UNSET
-if not isinstance({{ source }}, Unset):
+if not isinstance({{ source }}, Unset){%if query_parameter %} and {{ source }} is not None{% endif %}:
 {% if property.nullable %}
     {{ destination }} = {{ source }}.to_tuple() if {{ source }} else None
 {% else %}
diff --git a/openapi_python_client/templates/property_templates/list_property.pyi b/openapi_python_client/templates/property_templates/list_property.pyi
index d05a13960..317bb19e8 100644
--- a/openapi_python_client/templates/property_templates/list_property.pyi
+++ b/openapi_python_client/templates/property_templates/list_property.pyi
@@ -32,7 +32,7 @@ for {{ inner_source }} in {{ source }}:
 {% endmacro %}
 
 
-{% macro transform(property, source, destination, declare_type=True) %}
+{% macro transform(property, source, destination, declare_type=True, query_parameter=False) %}
 {% set inner_property = property.inner_property %}
 {% if property.required %}
 {% if property.nullable %}
@@ -45,7 +45,7 @@ else:
 {% endif %}
 {% else %}
 {{ destination }}{% if declare_type %}: Union[Unset, List[Any]]{% endif %} = UNSET
-if not isinstance({{ source }}, Unset):
+if not isinstance({{ source }}, Unset){%if query_parameter %} and {{ source }} is not None{% endif %}:
 {% if property.nullable %}
     if {{ source }} is None:
         {{ destination }} = None
diff --git a/openapi_python_client/templates/property_templates/model_property.pyi b/openapi_python_client/templates/property_templates/model_property.pyi
index e6746cb24..0ad78c991 100644
--- a/openapi_python_client/templates/property_templates/model_property.pyi
+++ b/openapi_python_client/templates/property_templates/model_property.pyi
@@ -15,7 +15,7 @@ if _{{ property.python_name }} is not None and not isinstance(_{{ property.pytho
 {% endif %}
 {% endmacro %}
 
-{% macro transform(property, source, destination, declare_type=True) %}
+{% macro transform(property, source, destination, declare_type=True, query_parameter=False) %}
 {% if property.required %}
 {% if property.nullable %}
 {{ destination }} = {{ source }}.to_dict() if {{ source }} else None
@@ -24,7 +24,7 @@ if _{{ property.python_name }} is not None and not isinstance(_{{ property.pytho
 {% endif %}
 {% else %}
 {{ destination }}{% if declare_type %}: Union[{% if property.nullable %}None, {% endif %}Unset, Dict[str, Any]]{% endif %} = UNSET
-if not isinstance({{ source }}, Unset):
+if not isinstance({{ source }}, Unset){%if query_parameter %} and {{ source }} is not None{% endif %}:
 {% if property.nullable %}
     {{ destination }} = {{ source }}.to_dict() if {{ source }} else None
 {% else %}
diff --git a/openapi_python_client/templates/property_templates/union_property.pyi b/openapi_python_client/templates/property_templates/union_property.pyi
index 4c632c60a..fadef57e5 100644
--- a/openapi_python_client/templates/property_templates/union_property.pyi
+++ b/openapi_python_client/templates/property_templates/union_property.pyi
@@ -24,11 +24,11 @@ def _parse_{{ property.python_name }}(data: Any) -> {{ property.get_type_string(
 {{ property.python_name }} = _parse_{{ property.python_name }}({{ source }})
 {% endmacro %}
 
-{% macro transform(property, source, destination, declare_type=True) %}
+{% macro transform(property, source, destination, declare_type=True, query_parameter=False) %}
 {% if not property.required %}
 {{ destination }}{% if declare_type %}: {{ property.get_type_string() }}{% endif %}
 
-if isinstance({{ source }}, Unset):
+if isinstance({{ source }}, Unset){%if query_parameter %} or {{ source }} is None{% endif %}:
     {{ destination }} = UNSET
 {% endif %}
 {% if property.nullable %}