iphone touch, only working in one scene

I have the following script:

 function Update () {
     for (var t = 0; t < Input.touchCount; ++t) {
         if (Input.GetTouch(t).phase == TouchPhase.Stationary) {
 
             var ray1 : Ray = Camera.main.ScreenPointToRay (Input.GetTouch(t).position);
             var hit1 : RaycastHit;
             
             if (Physics.Raycast (ray1,hit1)) {
                if(hit1.collider.gameObject.tag == "Untagged"){
                     Debug.Log("done");                
                }
             }
         }
     }
 }

in two scene's one scene it works fine, i tap on any object with the tag 'Untagged' and it Logs it 'done' as i want, but in another scene, same project, same script, when i tap an object tagged with 'Untagged, same as the other scene, it comes up with 'NullReferanceException' every time of the line

 var ray1 : Ray = Camera.main.ScreenPointToRay (Input.GetTouch(t).position);

i've put the script on different objects and everything, not that i can see that making a difference... doesn't work! as you might think... this is really annoying me! What's new with scripting?!

look forward to getting this sorted! thanks in advance for your help.