Seems to be working just fine when I test this in both 2.7 and 3.3! 

The reason this works is in the text I quoted in #134, that it's a `str` in 2.7 and a `bytestring` in 3.3. 

I could probably have rewritten it to ```if not isinstance(metadata, bytes):```, but from what I can see a negation isn't really needed here.