diff --git a/test/types/reply.test-d.ts b/test/types/reply.test-d.ts
index 61466db3b6..92318b33c8 100644
--- a/test/types/reply.test-d.ts
+++ b/test/types/reply.test-d.ts
@@ -42,8 +42,8 @@ const getHandler: RouteHandlerMethod = function (_request, reply) {
   expectType<(key: string) => boolean>(reply.hasTrailer)
   expectType<(key: string) => FastifyReply>(reply.removeTrailer)
   expectType<FastifyInstance>(reply.server)
-  expectAssignable<((httpStatus: string) => DefaultSerializationFunction)>(reply.getSerializationFunction)
-  expectAssignable<((schema: {[key: string]: unknown}) => DefaultSerializationFunction)>(reply.getSerializationFunction)
+  expectAssignable<((httpStatus: string) => DefaultSerializationFunction | undefined)>(reply.getSerializationFunction)
+  expectAssignable<((schema: {[key: string]: unknown}) => DefaultSerializationFunction | undefined)>(reply.getSerializationFunction)
   expectAssignable<((schema: {[key: string]: unknown}, httpStatus?: string) => DefaultSerializationFunction)>(reply.compileSerializationSchema)
   expectAssignable<((input: {[key: string]: unknown}, schema: {[key: string]: unknown}, httpStatus?: string) => unknown)>(reply.serializeInput)
   expectAssignable<((input: {[key: string]: unknown}, httpStatus: string) => unknown)>(reply.serializeInput)
diff --git a/types/reply.d.ts b/types/reply.d.ts
index dbb8e13ca9..d637f0868f 100644
--- a/types/reply.d.ts
+++ b/types/reply.d.ts
@@ -68,8 +68,8 @@ export interface FastifyReply<
   serializer(fn: (payload: any) => string): FastifyReply<RawServer, RawRequest, RawReply, RouteGeneric, ContextConfig, SchemaCompiler, TypeProvider>;
   serialize(payload: any): string | ArrayBuffer | Buffer;
   // Serialization Methods
-  getSerializationFunction(httpStatus: string, contentType?: string): (payload: {[key: string]: unknown}) => string;
-  getSerializationFunction(schema: {[key: string]: unknown}): (payload: {[key: string]: unknown}) => string;
+  getSerializationFunction(httpStatus: string, contentType?: string): ((payload: {[key: string]: unknown}) => string) | undefined;
+  getSerializationFunction(schema: {[key: string]: unknown}): ((payload: {[key: string]: unknown}) => string) | undefined;
   compileSerializationSchema(schema: {[key: string]: unknown}, httpStatus?: string, contentType?: string): (payload: {[key: string]: unknown}) => string;
   serializeInput(input: {[key: string]: unknown}, schema: {[key: string]: unknown}, httpStatus?: string, contentType?: string): string;
   serializeInput(input: {[key: string]: unknown}, httpStatus: string, contentType?: string): unknown;