Cannot invoke an object which is possibly 'undefined'.

Argument of type '(TsonTypeTesterCustom & TsonMarshaller<any, any>) | (TsonTypeTesterPrimitive & Partial<TsonMarshaller<any, any>>)' is not assignable to parameter of type '{ key: string; serialize: (arg: unknown) => SerializedType; }'.

Argument of type '(TsonTypeTesterCustom & TsonMarshaller<any, any>) | (TsonTypeTesterPrimitive & Partial<TsonMarshaller<any, any>>)' is not assignable to parameter of type '{ key: string; serialize: (arg: unknown) => SerializedType; }'.