Hide threads by usergroup

[Martin64]

Here's the error report the board sent me:

"Database error in vBulletin 2.2.8:

Invalid SQL:
SELECT

thread.threadid FROM thread
LEFT JOIN user ON (thread.postuserid = user.userid)
WHERE thread.forumid = 19
AND thread.sticky=0
AND user.usergroupid != 16
AND thread.visible=1
AND lastpost >= 1038763258 AND sticky=0

ORDER BY sticky DESC, lastpost DESC
LIMIT 0,20
mysql error: Column: 'lastpost' in where clause is ambiguous

mysql error number: 1052"