"unknown encoding" in senddigests

Traceback (most recent call last):
File "/usr/local/mailman/cron/senddigests", line 94, in ?
main()
File "/usr/local/mailman/cron/senddigests", line 86,
in main
mlist.send_digest_now()
File "/usr/local/mailman/Mailman/Digester.py", line
60, in send_digest_now
ToDigest.send_digests(self, mboxfp)
File
"/usr/local/mailman/Mailman/Handlers/ToDigest.py", line
123, in send_digests
send_i18n_digests(mlist, mboxfp)
File
"/usr/local/mailman/Mailman/Handlers/ToDigest.py", line
295, in send_i18n_digests
msg = scrubber(mlist, msg)
File
"/usr/local/mailman/Mailman/Handlers/Scrubber.py", line
257, in process
url = save_attachment(mlist, part, dir)
File
"/usr/local/mailman/Mailman/Handlers/Scrubber.py", line
348, in save_attachment
fnext = os.path.splitext(msg.get_filename(''))[1]
File "/usr/local/mailman/pythonlib/email/Message.py",
line 689, in get_filename
return unicode(newvalue[2], newvalue[0])
LookupError: unknown encoding