Actually, technically that shouldn't happen... The way Fortify Health works now, you can have ANY amount active. The way it kills you is that when the effect wears off, it subtracts the spells value from your Max HP and your Current HP. Therefore, if you take more damage than your natural health value during the spell's duration, you will drop dead when the effect wears off, completely nullifying the point of using the spell in the first place!
As a matter of fact, the way it is right now, the only logical reason that I can think of to ever use this spell is to make it so that you can survive an attack that would have otherwise killed you in one shot, and then healing before the Fortify Health effect wears off.
Please, I beg you, fix this problem!