I keep getting a "Target is immune" message when I try to cast Avid Boneyard a 2nd time in combat after it wears off. When this happens it simply will not let me cast it unless I can untarget any enemies that were hit by the first one. It doesn't always happen but it does often enough that it is annoying. I don't see anything about this skill that should cause an enemy to be immune for any reason. Any idea what is going on? (bug perhaps?)