How to drag a cube in orthographic camera?

Hi, I am trying to create a isometric game like clash of clans. I created a Terrain and I set my camera position to (0,300,-10) and Rotation to (40,45,0) and Perspective to Orthographic. I am using below code to drag a cube but when i drag the cube at some position cube is not able to visible or only some portion of cube is visible. It seems like position (X,Y,Z) all three are changing using below code. But i want to drag cube just like any top down game like Clash of Clans. Please help me to resolve my issue.

 void OnMouseDrag ()
 {        
    Vector3 mousePosition = new Vector3 (Input.mousePosition.x, Input.mousePosition.y, 0);
    Vector3 objPosition = Camera.main.ScreenToWorldPoint (mousePosition);                            
    this.target.transform.position = objPosition;
                             
 }