@senthil: I'm not quite sure what your sentence referencing couchdb is getting at, but the headers that are being queried are in an email.Message object, and contain only string values unless some code is misusing the API and setting new non-string values after the initial header parse is has been done.

Regardless, though, my suggested change makes the code work with the documented API (which strongly implies that default is a string) while continuing to accept any previously working values that existing code may be passing in.