I'm getting an Error when guests post.

[prettyboi]

Okay, here's the problem! I'm getting this error report when my guests post in one particular forum only that they are allowed to.

Database error in vBulletin 3.5.1:

Invalid SQL:
SELECT COUNT(*) AS count FROM vb_user WHERE referrerid =;

MySQL Error : You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near '' at line 1
Error Number : 1064
Date : Sunday, March 26th 2006 @ 01:08:55 AM
Script : http://www.asianunityforum.com/showt...=3895#post3895
Referrer :
IP Address : 68.230.237.222
Username : PRETTYBOI
Classname : vb_database

Can someone help me fix this please or give me an MySQL commands to execute this query to correct the problem. Thanks!!!