This error is triggered for a required parameter that's missing.  

There's currently a test in test_reqparse.py called test_parse_required() that looks for the BadRequest exception but it doesn't look at or print the body of the error message.

You should probably write a few new tests that make sure single location and multiple location values have the right message data.  Something like this would probably work:

        req = Request.from_values("/bubble")

        parser = RequestParser()
        parser.add_argument("foo", required=True)

        message = ''
        try:
            parser.parse_args(req)
        except exceptions.BadRequest as e:
            message = e.data['message']
        
        self.assertEquals(message, 'foo is required in values')