When you send JSON data into a Slim Framework application with a content-type of application/json, then Slim will decode it for you if you use getParsedBody(). [...] If there's an error however, you get [an empty response]. If you care about this, you can use json_last_error_msg() and return an error.
He includes a code example of using this function on an endpoint, providing more information on why the JSON parsing failed. He also includes a "bonus" that makes use of the jsonlint library to inspect the JSON message and provide back a more human-readable message letting the user know what exactly was wrong with the JSON they provided.