BARK UP THE WRONG TREE

{v. phr.}, {informal} To choose the wrong person to deal with or the wrong course of action; mistake an aim. * /If he thinks he can fool me, he is barking up the wrong tree./ * /He is barking up the wrong tree when he blames his troubles on bad luck./ * /The police were looking for a tall thin man, but were barking up the wrong tree; the thief was short and fat./