"Invalid location" when casting Necrotic Orb

ectoplasmicninja

I've only recently finally started using orb in my rotation so it's still the base version. A good percentage of the time when I hit the button to cast it, it doesn't fire and a message pops up in the top right saying "invalid location". Orb doesn't require a target, it's an area ability, so it can't be a matter of my cursor not being on an enemy. It happens several times in every dungeon run. I have been having problems with skills failing to fire generally but this one actually throws an error message. Anyone else have this issue?