BARK UP THE WRONG TREE

"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 can not fool me; I know him too well.)
He is barking up the wrong tree
when he blames his troubles on bad luck.
(His troubles come from his own mistakes, not luck.)
The police were looking for a tall thin man,
but were barking up the wrong tree; the thief was short and fat.
(The police were hunting the wrong man.)