[Dovecot] Index corruption

I've been seeing this in my log (test69) for a user who has Thunderbird as a client:

Error: IMAP(andrew): Corrupted index file /home/a/andrew/Maildir/dovecot.index: header doesn't end with NUL

Thoughts?

-D

what're the units of the coefficient of agnosticity? I don't knows per hour?