2D game look at mouse rotate

Hi. Iv'e been trying to make this work all day, and would greatly appreciate some help! Im making a 2D birds eye view game. There is a sprite with a script attached, and I want him to rotate on the Z axis to point at my mouse cursor. Right now with my code though, he becomes invisible to the camera because t=he starts rotating on the Y and X axis! Please help much confused, thanks. CODE:

 //vector for current mouse position(works)
 var mouse = Vector2(Input.mousePosition.x,Input.mousePosition.y); 
     Debug.Log(mouse);
 //does something..
     var relativePos =  mouse - transform.position;
 //sets sprite rotation to relativePos
     transform.rotation = Quaternion.LookRotation(relativePos);