Rotating sprite with mouse

Hello, I have used the rotating code made by Aarlangdi (https://www.youtube.com/watch?v=PW42etn0PBw) Thanks to him. It works quite well with 3D object such as cube, but it cant work on sprite. Anyone know why is it? Appreciate much!

using UnityEngine; using System.Collections;

public class rotate : MonoBehaviour {

 void OnMouseDown(){
    Vector3 pos = Camera.main.WorldToScreenPoint(transform.position);
    pos = Input.mousePosition - pos;
    baseAngle = Mathf.Atan2(pos.y, pos.x) * Mathf.Rad2Deg;
    baseAngle -= Mathf.Atan2(transform.right.y, transform.right.x) *Mathf.Rad2Deg;
 }

 void OnMouseDrag(){
     Vector3 pos = Camera.main.WorldToScreenPoint(transform.position);
     pos = Input.mousePosition - pos;
     float ang = Mathf.Atan2(pos.y, pos.x) *Mathf.Rad2Deg - baseAngle;
     transform.rotation = Quaternion.AngleAxis(ang, Vector3.forward);
 }

}