Core Data migration, missing source managed object model. Help!

I've created two managed object models, now residing in a momd, and I've created the mapping model. I set the new version to be the current version, and I've enabled migrations by altering the options dictionary in my app delegate's -persistantStoreCoordinator. But, when I launch the app, I get "Persistent store migration failed, missing source managed object model." It sounds like a configuration issue, but I can't figure out where I went wrong.