Getting Database Error Upon Deleting Threads

[Vibhor]

I’m getting the following database error upon physically deleting more than one threads (which are already soft deleted).

Invalid SQL:

UPDATE thread
SET attach = attach -
CASE
WHEN threadid = 13339 THEN 1
ELSE 0
END
WHERE threadid IN (-1,13339);

MySQL Error : BIGINT UNSIGNED value is out of range in '(`database_name`.`thread`.`attach` - (case when (`database_name`.`thread`.`threadid` = 13339) then 1 else 0 end))'
Error Number : 1690
Request Date : Friday, June 28th 2013 @ 10:18:23 PM
Error Date : Friday, June 28th 2013 @ 10:18:23 PM
Script : http://www.xxxx.com/inlinemod.php?do...39,13337,13336
Referrer : www.xxxxx.com/inlinemod.php
Classname : vB_Database

Can someone help to pinpoint on the exact issue?

Thanks.