Corpse Plague

Hey, I'm one of the few SK's around (Shillien Knight) and I just obtained a new skill learned called Corpse Plague. The problem is everytime I try to use it, it tells me "incorrect target." Can someone explain or email me how this skill works, so that I can begin using it to its effects, thanks?

After you kill a target you can use the skill on the corpse to make it explode and send a poison cloud to nearby targets. Good for those AOE parties or when you pull too much aggro, but pretty useless as a main skill.