| This is probably very bad news, but there are two reasons you get that message. One is that you really cast it on an invalid object. The other is that after you cast it on one NPC, you must either kill the NPC or let the spell expire before you cast it again. If you cast it on two different living characters at the same time, it breaks the quest permanently. The symptom after that happens is you always get the message you mentioned, so unless you've been going around casting it on goblins and treasure chests I'm guessing that's your problem.
You can fix it with a console code on the PC, but on the 360 the only fix is to go back to a save game before you cast the glitch occurred.
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