AWS Lambda Server-Sent Events on dotnet with AWS API Gateway

AWS has recently announced support for response streaming in AWS API Gateway. I am using AWS Lambda on dotnet with ASP.NET using following pattern

public class LambdaEntryPoint : APIGatewayProxyFunction { protected override void Init(IWebHostBuilder builder) { builder .UseStartup<Startup>(); } }

This works when using Buffered integration type between AWS API Gateway and AWS Lambda, however once integration type is changed to Stream then I have

{"message": "Internal server error"}

Is there some error on my end?

Thanks