[Dovecot] assertion failed: (fd >= 0)

[Jos Chrispijn]

After I activated disabling POP3 for one of my users (not user 'jos'), 
the following is in my logfile:

Dec  3 16:03:33 webrz dovecot: IMAP(jos): Disconnected
Dec  3 16:03:36 webrz dovecot: IMAP(jos): Disconnected in IDLE
Dec  3 16:03:36 webrz dovecot: IMAP(jos): file ioloop.c: line 22 
(io_add): assertion failed: (fd >= 0)
Dec  3 16:03:36 webrz dovecot: child 74197 (imap) killed with signal 6

Can someone tell me what is going wrong?
Have a deny.imap file with no users in it and a deny.pop3 file with one 
user in it...

Jos