Ur-priest has as a prerequisite an inability to cast divine spells. It also grants divine spells. The first level's fine, because you don't necessarily have spells yet, but every level after that the class would seem to interfere with its own progression. Is there some rule that resolves this issue?
I'm aware that there are lots of other ways to level up a class other than actually putting levels in it, but this is about doing it the 'normal' way.