[eluser]boltsabre[/eluser]
how are you calling your model function _validateFormFields?
This is just a guess, but because it's a private method, if you try to call it direct from your controller you'll run into problems, however, if it was to be called from inside your model from another method it should work.
I'm at work atm so cannot test it, I'll leave it up to you, please report back if it now works or not when you call it from another method inside the same model.