Uncaught recipients.remove not in list in Post (1.0b11)

I got the error page below when trying to approve a message for posting. The message was submitted with From: address which doesn't match a subscriber address, which was why it was sent to me for approval (the list has no posting from nonmembers).
Ian.
Bug in Mailman version 1.0b11
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Traceback:
Traceback (innermost last): File "/var/lib/mailman/scripts/driver", line 112, in run_main main() File "/usr/lib/mailman/Mailman/Cgi/admindb.py", line 124, in main HandleRequests(doc) File "/usr/lib/mailman/Mailman/Cgi/admindb.py", line 214, in HandleRequests list.HandleRequest(request, v) File "/usr/lib/mailman/Mailman/ListAdmin.py", line 122, in HandleRequest self.HandlePostRequest(request_data[2:], value, comment) File "/usr/lib/mailman/Mailman/ListAdmin.py", line 131, in HandlePostRequest self.Post(msg, 1) File "/usr/lib/mailman/Mailman/MailList.py", line 1236, in Post recipients.remove(self.GetUserSubscribedAddress(sender)) ValueError: list.remove(x): x not in list
participants (1)
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ijackson@chiark.greenend.org.uk