How do you tell the procedures that the entity is a player?

So I'm making an enchantment that when a helmet with this enchant is being worn by a player, they have night vision until either the helmet is taken off, or the helmet breaks. But, it won't work because the "element's don't compile correctly". My only guess is that it needs to know what entity is being effected. So how would I tell it that said entity is a player?