diff --git a/src/types/serialize.ts b/src/types/serialize.ts
index c971bc6c82..2442560a3d 100644
--- a/src/types/serialize.ts
+++ b/src/types/serialize.ts
@@ -28,7 +28,7 @@ type FilterKeys<TObj extends object, TFilter> = {
 // prettier-ignore
 export type Serialize<T> =
  IsAny<T> extends true ? any :
- T extends JsonPrimitive ? T :
+ T extends JsonPrimitive | undefined ? T :
  T extends Map<any,any> | Set<any> ? Record<string, never> :
  T extends NonJsonPrimitive ? never :
  T extends { toJSON(): infer U } ? U :
diff --git a/test/fixture/types.ts b/test/fixture/types.ts
index a727546446..9e2624344f 100644
--- a/test/fixture/types.ts
+++ b/test/fixture/types.ts
@@ -311,7 +311,14 @@ describe("defineCachedEventHandler", () => {
 
 describe("type helpers", () => {
   it("Serialize", () => {
+    expectTypeOf<Serialize<undefined>>().toEqualTypeOf<undefined>();
+    expectTypeOf<Serialize<{ test?: string }>>().toEqualTypeOf<{
+      test?: string;
+    }>();
     expectTypeOf<Serialize<{ test: Date }>>().toEqualTypeOf<{ test: string }>();
+    expectTypeOf<Serialize<{ test?: Date }>>().toEqualTypeOf<{
+      test?: string;
+    }>();
     expectTypeOf<Serialize<{ test: Map<string, string> }>>().toEqualTypeOf<{
       test: Record<string, never>;
     }>();