An error occurred (InvalidSsmlException) when calling the StartSpeechSynthesisTask operation: Invalid SSML request

Why does the response in case of an error omit any details like which bits of the provided SSML is invalid?

I'm currently facing a weird situation: When I try to start a StartSpeechSynthesisTask from a python application, using boto3, I get an InvalidSsmlException. But when I try the same input in the AWS Polly > Text-to-Speech console, it works flawlessly.

Bet if the Exception included more info I would find the answer on my own