Database Error

[PHILLYFAN]

Out of the blue, when people register to my site they fill out everything and hit send the white data base error screen comes up and then i get this in my email

Database error in vBulletin 3.7.3:

Invalid SQL:
INSERT INTO userlist (userid, relationid, type, friend) VALUES (420, , 'buddy', 'yes');

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 ' 'buddy', 'yes')' at line 1
Error Number : 1064
Request Date : Friday, January 16th 2009 @ 09:42:06 PM
Error Date : Friday, January 16th 2009 @ 09:42:07 PM
Classname : vB_Database
MySQL Version :

Anyone know what I can do to fix this?
Thanks!