Redmine

If you try to remove an assignee from a ticket, you can either use the bulk or detail view. In both cases, changing the assignee to <<nobody>> or " " will not work.
There seems to be a missing assigned_to_id field in the UPDATE statement:

   (1.6ms)  UPDATE `issues` SET `updated_on` = '2014-06-13 13:48:57', `lock_version` = 11 WHERE (`issues`.`id` = 350 AND `issues`.`lock_version` = 10)

if you assign the ticket to a valid user, this happens:

  (1.3ms)  UPDATE `issues` SET `assigned_to_id` = 3, `updated_on` = '2014-06-13 13:47:15', `lock_version` = 3 WHERE (`issues`.`id` = 217 AND `issues`.`lock_version` = 2)

Environment:
Redmine version 2.5.1.stable
Ruby version 2.0.0-p457 (2014-03-03) [i386-linux-gnu]
Rails version 3.2.17
Environment production
Database adapter Mysql2