Inherent Stone Skin

So not sure if this is a bug or by design but it seems when you inherent stone skin and succeed it takes no mana. But if your mana is too low to cast stone skin, ie 40 mana, then you can not inherent stone skin. In addition when you fail to inherent stone skin it takes half mana as if you failed to cast the spell. Just seemed a little wierd. I could get a log if needed. Thanks