diff --git a/graphql/type/definition.py b/graphql/type/definition.py
index 2f499aec..52128aea 100644
--- a/graphql/type/definition.py
+++ b/graphql/type/definition.py
@@ -551,7 +551,10 @@ def parse_literal(self, value_ast):
     @cached_property
     def _value_lookup(self):
         # type: () -> Dict[str, GraphQLEnumValue]
-        return {value.value: value for value in self.values}
+        return {
+            value.value.value if isinstance(value.value, PyEnum) else value.value: value
+            for value in self.values
+        }
 
     @cached_property
     def _name_lookup(self):
diff --git a/graphql/type/tests/test_serialization.py b/graphql/type/tests/test_serialization.py
index d91fd29d..85492b26 100644
--- a/graphql/type/tests/test_serialization.py
+++ b/graphql/type/tests/test_serialization.py
@@ -81,3 +81,24 @@ class Color(Enum):
     assert enum_type.serialize(Color.RED.value) == "RED"
     assert enum_type.serialize(Color.EXTRA) is None
     assert enum_type.serialize(Color.EXTRA.value) is None
+
+
+def test_serialize_enum_pyenum():
+    class Color(Enum):
+        RED = 1
+        GREEN = 2
+        BLUE = 3
+        EXTRA = 4
+
+    enum_type = GraphQLEnumType(
+        "Color",
+        values={
+            "RED": GraphQLEnumValue(Color.RED),
+            "GREEN": GraphQLEnumValue(Color.GREEN),
+            "BLUE": GraphQLEnumValue(Color.BLUE),
+        },
+    )
+    assert enum_type.serialize(Color.RED) == "RED"
+    assert enum_type.serialize(Color.RED.value) == "RED"
+    assert enum_type.serialize(Color.EXTRA) is None
+    assert enum_type.serialize(Color.EXTRA.value) is None