Ultrasphinx search error... Invalid class name

I've got Ultrasphinx powering the search on my Rails app. It's working
with all of my models when they're first indexed, but then, after what
seems like a random amount of time, I'll get the following error on
just *one* of my models:

search#show (Ultrasphinx::UsageError) "Invalid class name \"Playlist

I set up this model like so:


...and like I said, it works just fine off the bat. But eventually
something goes wrong and I get the error above. Any ideas what could
be happening here?

Any suggestions on this one? Still having this problem, not sure where
to start troubleshooting.