Ahhh... thanks Jorg. I have both lines in my forums - the line that I put in the hack is the line for the private message preview hack. That's why I never had this problem.
When I wrote the installation file, I deleted the standard line and kept the wrong line for this hack.

I meant to delete that one, not the one that is standard.
I've fixed the installation text file, and sent an update to all who installed this so they know to fix it.