API gateway returns 500 internal server error with message null with java authorizer

Question

Create AWS authorizer lambda in java to do token validation and create iam policy based on permission. Working as expected when we run using aws jar in aws env. When i converted to native image using graalvm getting error message as below with 500 internal status

{ "message": null }

Removed all validation logic and created hardcoded json policy using JSONObject still no luck, getting same issue. Below is the policy which is generated by lambda

{
    "policyDocument": {
        "Version": "2012-10-17",
        "Statement": [
            {
                "Action": "execute-api:Invoke",
                "Resource": "arn:aws:execute-api:region:accountid:apiid/stage/GET/*",
                "Effect": "Allow"
            }
        ]
    },
    "context": {
        "sub": "Sample"
    },
    "principalId": null
}

Same policy works fine for normal jar and deployed using java runtime env.

With help of API gateway logs found out the authorizer response is sent as APIGatewayProxyResponseEvent where I am sending Iampolicy pojo, where API gateway is throwing exception saying Execution failed due to configuration error: Invalid JSON in response: Unrecognized field "isBase64Encoded" , not marked as ignorable.

Please some input/suggestion will be helpful.