Ray error with touches.position

Hi,

I have a little piece of code, to make a raycast on the position you touch the screen (iPhone). I used it in another script, on the exact same way. But now i get an error..

 // Buttons
             if (Input.touches[i].phase == TouchPhase.Began)
             {
                 RaycastHit hit;
                 Ray ray = Camera.main.ScreenPointToRay(Input.touches[i].position);
                 if (Physics.Raycast(ray, out hit, 100, layermaskButtons))
                 {
                     print("test");
                 }
             }

The error is:

NullReferenceExeption: Object reference is not set to an instance of an object

When i dubble click the error, it says that this line is wrong:

Ray ray = Camera.main.ScreenPointToRay(Input.touches[i].position);

But I used that exact same line in another script, and that works perfectly!