AWS IoT Custom Authorizer Not working for HTTP GET GetRetainedMessage

I have implemented a custom authorizer for my AWS IoT and it is working fine for publishing messages. But returns "Internal Server Error" if I tried to fetch retained messages via HTTPS. Policies seems to be fine and the lambda function is executing correctly as per the logs. Any help is much appreciated.

{ "message": "Internal Server Error", "traceId": "975693ad-b482-2e21-8686-60f8392cfd4a" }