If the message contains 8bit bytes in a header, then getitem is going to return a Header object.  decode_header does not operate on Header objects, as you have observed.  Thinking about it some more, having decode_header operate on a Header and return its chunks is a decent binary interface for Header.  This is right on the borderline between a feature and a bug fix, but given that getitem returning Header after a parse is a new feature in 3.2, I think I'm going to treat it as a bug that decode_header doesn't handle that case.