physics.raycast nullReferenceException

i get this error when using this raycast code:

 var back = transform.TransformDirection (Vector3.back);
 var hit : RaycastHit;
      if (Physics.Raycast (transform.position, back, 10)) {
      print (hit.collider.tag);
      }

the error specifically is :

NullReferenceException: Object reference not set to an instance of an object.

the object it hits is a prefab which has been instantiated and has a box collider.