It seems the current code simply forgot to break on indexed types and
therefore hit some g_return_*if_reached() code breaking the logics.
Looking further, I see some code taking care of indexed images and
converting them to RGB. And testing after adding breaks looks like it
works just fine.
So I am assuming this was just forgotten breaks indeed, and not on
purpose not allowing indexed images (if that were the intent though,
this is not how it should be done).