missing GID

 tail -f dovecot.log
May 27 00:46:17 master: Warning: Killed with signal 15 (by pid=31385 uid=0 code=kill)
May 27 00:46:30 imap(jason): Error: user jason: Couldn't drop privileges: User is missing GID (see ma                                                                                                          il_gid setting)
May 27 00:46:30 imap(jason): Error: Internal error occurred. Refer to server log for more information