[Dovecot] mbox originally needed 0 bytes, now needs maxint bytes

I'm running dovecot 1.2.16 from the ports collection on FreeBSD 8.1-STABLE, amd64.

I've long been getting the following kind of messages in my logs, on random occasions when I access or move mail into mboxes:

Aug 16 00:43:58 myhost dovecot: IMAP(mike): mbox /home/mike/mail/feedback: seq=88 uid=91 uid_broken=0 originally needed 0 bytes, now needs 18446744073709551615 bytes

Whatever I'm trying to do always succeeds, but apparently dovecot is confused about something. That number (2^64-1) suggests it has to do with the amd64 architecture. Did a compile option get overlooked?

Thanks for whatever you can suggest. Let me know if you need more info.