If the handler function throws an error, the execution of lambda_call never returns. A stack trace is printed containing the message TypeError: Object of type FrameSummary is not JSON serializable.

Looks like this is caused in line 127 of main.py (where you're setting stackTrace):

        result = json.dumps({
            "errorMessage": str(err[1]),
            "stackTrace": traceback.extract_tb(err[2]),
            "errorType": err[0].__name__
        }, indent=4, separators=(',', ': '))

json.dumps is not happy with serializing an array of FrameSummary objects, which is what extract_tb returns.