None body in parsed mails

[emilienschultz]

I try to use the Single Word Trend notebook example and raise an error when iterating across the archive. Some mails (seems to be multipart mails) have a None body content. It seems it comes from the get_text function in bigband package, more especially in the multipart section of the if test. For instance, the email "[email protected]" in the file February 2013 has a None body where you should have some text.