Problems with a bear spell

I have a hero spell based of metamorphosis that changes the user into a bear. Each time i activate it, it works fine until the duration in the spell runs out and then i get a fatal error before he can change back into regular form. Should i just base the spell off storm, earth, and fire or does anyone know what the problem might be? Thx

I would say just base it off of Storm, earth and fire, because you'll get the same effect anyway, and you can make it look cooler.