BIGINT UNSIGNED value is out of range error

[Boofo]

I just got the following error while deleting a thread:

Database error in vBulletin 4.1.12:

Invalid SQL:
UPDATE user SET
	totalwords = totalwords - 6
WHERE userid = 61;

MySQL Error   : BIGINT UNSIGNED value is out of range in '(`xxxxx_xxxxxxxxx`.`user`.`totalwords` - 6)'
Error Number  : 1690
Request Date  : Friday, May 18th 2012 @ 07:49:07 AM
Error Date    : Friday, May 18th 2012 @ 07:49:07 AM
Script        : http://www.iamgrumpy.com/forums/postings.php?do=dodeletethread&threadid=362
Referrer      : http://www.iamgrumpy.com/forums/postings.php
IP Address    : xxx.xx.xx.xxx
Username      : Boofo
Classname     : vB_Database
MySQL Version : 5.5.22-cll

It has to do with the following code but I'm not sure how to fix it. I have never run into this error before.

Hook: threaddata_delete

if ($physicaldel)
{
    $threadinfo = fetch_threadinfo($threadid);
    $postinfo = fetch_postinfo($threadinfo['firstpostid']);
    $userdm =& datamanager_init('User', $this->registry, ERRTYPE_SILENT);
    $userdm->set_existing($postinfo);
    $userdm->set('totalwords', 'totalwords - ' . str_word_count($postinfo['pagetext']), false);
    $userdm->save();
    unset($userdm);

    $posts = $this->dbobject->query_read("
        SELECT post.postid
        FROM " . TABLE_PREFIX . "post AS post
        WHERE post.threadid = $threadid
    ");

    while ($post = $this->dbobject->fetch_array($posts))
    {
        $postinfo = fetch_postinfo($post['postid']);
        $userdm =& datamanager_init('User', $this->registry, ERRTYPE_SILENT);
        $userdm->set_existing($postinfo);
        $userdm->set('totalwords', 'totalwords - ' . str_word_count($postinfo['pagetext']), false);
        $userdm->save();
        unset($userdm);
    }
}

Does anyone see what I am missing there?