GuiText instantiation at position?

Hi Everybody! I'm doing the Pong game and I just wanted to know why this code doesn't work! I have a prefab that is my GameObject with a GUIText component in it and I just want it to instantiate on the screen based on the ball's position. So here's what I have:

    void OnCollisionEnter(Collision col){
        if(col!=null){
           Vector3 a = Camera.mainCamera.WorldToViewportPoint(transform.position);
           Object s = GameObject.Instantiate (Prefab,a,Quaternion.identity);
        }
     }

The reason why I have a prefab is that I want the GUIText to play an animation next spawned, then it just kill himself after the animation. The problem is that any GUIText that I spawn, it spawns on (1,0) coordinates on the screen, but the WolrdToViewportPoint coordinates are right! Any help? Thanks in advance!