"value must be enumerable" exception when using a sweeper?

Folks,

I'm going slightly crazy here - I implemented a Sweeper in an older app
and used nearly the same exact code in my new app, yet I keep getting
this 'value must be enumerable' exception. Even after I cut most of the
code away, I still get the error. If anyone can help, I would be much
obliged.

Notes: I'm using Engines... wonder if that could cause a problem. And
yet, if I 'observe' my User model, which is part of my main app and not
an engine or plugin, it still gives me the same exception.

Thanks!