Thread:Extension talk:User Merge and Delete/Help: "Fatal error: Call to a member function isListed() on a non-object"

I just installed this on my wiki, and I'm getting the following error.
 * Fatal error: Call to a member function isListed on a non-object in /home/wepwtorg/public_html/wiki/includes/SpecialPage.php on line 210

How can I fix this?

- Carissa