Private Messages not opening - mysql error

When I access my PM inbox from the tapatalk app I can see a listing of my PM's, as well as the summary of who sent it and the first line of the PM.

When I try to open the PM, it won't open, it just shows blank.

My forum's SQL server then sends me an email with the following error:

Code:

Database error in vBulletin 3.8.1:

Invalid SQL:
SELECT gfpoforumoptions FROM vbwdfforum WHERE forumid =;

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
Request Date  : Wednesday, November 11th 2009 @ 02:47:06 PM
Error Date    : Wednesday, November 11th 2009 @ 02:47:06 PM
Script        : http://www.<forumname>.com/vBulletin/mobiquo/mobiquo.php
Referrer      : 
IP Address    : 70.71.223.180
Username      : <redacted>
Classname     : vB_Database
MySQL Version : 5.0.81-community-log

How do I fix this? Everything else seems to work okay.

Thanks!