cannot concatenate 'str' and 'NoneType' objects


Traceback (most recent call last):
  File "/mnt/hd/web/1bpm.net/ml.csound/app/handler.py", line 386, in <module>
    handle()
  File "/mnt/hd/web/1bpm.net/ml.csound/app/handler.py", line 379, in handle
    return Attachment(mailing_list).render({"ident": parts[2]})
  File "/mnt/hd/web/1bpm.net/ml.csound/app/handler.py", line 294, in render
    parsed = ParsedMessage("\n".join(result[3]), int(idents[1]))
  File "/mnt/hd/web/1bpm.net/ml.csound/app/handler.py", line 39, in __init__
    self.parse(msg)
  File "/mnt/hd/web/1bpm.net/ml.csound/app/handler.py", line 72, in parse
    self.attachment += part.get_payload(decode=True)
TypeError: cannot concatenate 'str' and 'NoneType' objects