diff --git a/Sources/PostgREST/PostgrestQueryBuilder.swift b/Sources/PostgREST/PostgrestQueryBuilder.swift
index 4924b15..c5deeaf 100644
--- a/Sources/PostgREST/PostgrestQueryBuilder.swift
+++ b/Sources/PostgREST/PostgrestQueryBuilder.swift
@@ -79,14 +79,13 @@ public final class PostgrestQueryBuilder: PostgrestBuilder {
     onConflict: String? = nil,
     returning: PostgrestReturningOptions = .representation,
     count: CountOption? = nil,
-    ignoreDuplicates: Bool? = nil
+    ignoreDuplicates: Bool = false
   ) -> PostgrestFilterBuilder {
     method = "POST"
     var prefersHeaders = [
-      ignoreDuplicates.map { "resolution=\($0 ? "ignore" : "merge")-duplicates" },
+      "resolution=\(ignoreDuplicates ? "ignore" : "merge")-duplicates",
       "return=\(returning.rawValue)",
     ]
-    .compactMap { $0 }
     if let onConflict = onConflict {
       appendSearchParams(name: "on_conflict", value: onConflict)
     }
diff --git a/Tests/PostgRESTTests/BuildURLRequestTests.swift b/Tests/PostgRESTTests/BuildURLRequestTests.swift
index b252e5b..b7e851a 100644
--- a/Tests/PostgRESTTests/BuildURLRequestTests.swift
+++ b/Tests/PostgRESTTests/BuildURLRequestTests.swift
@@ -78,6 +78,14 @@
             .select()
             .contains(column: "name", value: ["is:online", "faction:red"])
         },
+        TestCase(name: "test upsert not ignoring duplicates") { client in
+          client.from("users")
+            .upsert(values: ["email": "johndoe@supabase.io"])
+        },
+        TestCase(name: "test upsert ignoring duplicates") { client in
+          client.from("users")
+            .upsert(values: ["email": "johndoe@supabase.io"], ignoreDuplicates: true)
+        },
       ]
 
       for testCase in testCases {
diff --git a/Tests/PostgRESTTests/__Snapshots__/BuildURLRequestTests/testBuildRequest.test-upsert-ignoring-duplicates.txt b/Tests/PostgRESTTests/__Snapshots__/BuildURLRequestTests/testBuildRequest.test-upsert-ignoring-duplicates.txt
new file mode 100644
index 0000000..c5567bf
--- /dev/null
+++ b/Tests/PostgRESTTests/__Snapshots__/BuildURLRequestTests/testBuildRequest.test-upsert-ignoring-duplicates.txt
@@ -0,0 +1,7 @@
+curl \
+	--request POST \
+	--header "Accept: application/json" \
+	--header "Content-Type: application/json" \
+	--header "Prefer: resolution=ignore-duplicates,return=representation" \
+	--data "{\"email\":\"johndoe@supabase.io\"}" \
+	"https://example.supabase.co/users"
\ No newline at end of file
diff --git a/Tests/PostgRESTTests/__Snapshots__/BuildURLRequestTests/testBuildRequest.test-upsert-not-ignoring-duplicates.txt b/Tests/PostgRESTTests/__Snapshots__/BuildURLRequestTests/testBuildRequest.test-upsert-not-ignoring-duplicates.txt
new file mode 100644
index 0000000..052aa50
--- /dev/null
+++ b/Tests/PostgRESTTests/__Snapshots__/BuildURLRequestTests/testBuildRequest.test-upsert-not-ignoring-duplicates.txt
@@ -0,0 +1,7 @@
+curl \
+	--request POST \
+	--header "Accept: application/json" \
+	--header "Content-Type: application/json" \
+	--header "Prefer: resolution=merge-duplicates,return=representation" \
+	--data "{\"email\":\"johndoe@supabase.io\"}" \
+	"https://example.supabase.co/users"
\ No newline at end of file