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Thanks for the report and patch suggestion, but...
This is actually the way it is designed to work. get_payload(i) returns the ith element of a multipart payload. Your workaround is in fact the correct way to do this operation. decode=True is documented to return None if is_multipart is True.
You will note that if decode=False, you get back the Message sub-object, not a string. Since decoding a Message object (as opposed to its payload) is not a meaningful operation, None is returned for decode=True. Arguably we should raise a TypeError or ValueError instead, but we don't for historical reasons.