Error in L4D2

[Shyria Dracnoir]

Whenever I try to pull up The Passing campaign in single player mode, the loading screen progresses normally, but right before it starts my game shuts down completely and gives me an error message that looks like this:

5/-survivor_bot: UTIL_SetModel: not precached: models/survivors/survivor_biker.mdl

I'm no programmer, but I'm guessing it has to do with problems loading the bot survivors (pretty obviously Francis from the nickname, though once it gave me something else right at the end that I didn't write down. I've gotten this version twice though). One of my friends just gave me a bunch of custom maps and textures the other day, and the first time I noticed this error was after I put them in (copy/pasted from a portable harddrive into the addons folder) though it might be the problem's been around for longer and me checking then was a coincidence. Any thoughts?

EDIT: I've been able to join games in Online Campaigns just fine, though I haven't tried starting one myself (and booting up Versus might be moot since the bots don't show up there anyways).