Can't delete non-active users from Page 1

I have always been able to go into my forum’s non-active registered users list and delete spammers. Lately, when I try to delete a user I get the following message:
Could not delete group for this user


SQL Error : 1064 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 2

DELETE FROM phpbb_groups WHERE group_id =

Line : 421
File : memberlist.php

I only get this message when I attempt to delete from page 1. I can delete from others pages just fine. Has anyone seen this before? I am not a phpbb expert, but I have searched around and cannot find this problem elsewhere. My phpbb version is 2.0.19, and the memberlist file version is v 2005/09/14 18:14:30. Thank you