invocation_response::failure does not work with HTTP API Gateway ?

I'm using the HTTP API Gateway, my c++ runtime is built with amazonlinux:latest in a docker image.
All responses that return with invocation_response::failure return an Error 500, Internal Server Error.
```
#include
using namespace aws::lambda_runtime;
static invocation_response my_handler(invocation_request const& req)
{
return invocation_response::failure("error message here"/*error_message*/, "error type here" /*error_type*/);
}

int main()
{
run_handler(my_handler);
return 0;
}
```
However, when I use invocation_response::success, the message arrives on my requesting client, but I ofc. can only send json objects back. (Anyone got an idea why that is ?)

I read everything google found and digged through all kinds of files and aws docus, I could not find out where the problem is. Could someone please give me a hint if the args have to match some specific ones or what I am doing wrong ?
Thanks!