Problem with Camera.mainCamera.ScreenPointToRay(Input.mousePosition);

I want to get the point, where my mouse is pointing in wold space. But i get this error:

NullReferenceException

UnityEngine.Camera.ScreenPointToRay (Vector3 position) this is my script: function Update () { var hit : RaycastHit; var ray : Ray = Camera.mainCamera.ScreenPointToRay(Input.mousePosition);
if(Physics.Raycast(ray, hit, Mathf.Infinity)) { Debug.Log(hit.point);
}
}
The Console says the error is in this line: var ray : Ray = Camera.mainCamera.ScreenPointToRay(Input.mousePosition); ... i dont know what is false?! Can someone help me?