Currently, Death Pact always says "can't target creeps of this level", no matter what the target is, as long as the target has a too high level.

This includes heroes. If you try to cast it on a level 7 hero, it gives the creep level error message. If you target a hero below level 7, it gives the proper hero error message.

This also goes for other units that are invalid targets, like creep-heroes (gives creep-hero error message on e.g. Familiars, gives creep level error message on Primal Split spirits)