Perdo Ignem & Corpus

In the case of Wizards Icy Grip (PeIg30) why do you not require a Corpus requisite?

Because you're destroying heat in a given area. It so happens that the given area is described by a corpus entity, but there's not any restriction of using this spell against an animal. Also it is the Form of Ignem that is being destroyed (heat) and not anything Corpus or Animal.