Lynda Tutroial projectile error

Ok so im doing the lynda tutorial for unity using unity 4 and in the tut she tells me to make this code

 #pragma strict
 
 var projectile : GameObject;
 var fireRate : float = 0.5;
 internal var nextFire : float;
 var speed : float = 5;
 
 function Start () {
 
 }
 
 function Update () {
 
 // fire1 is left mouse or left control
 
 if (Input.GetButton ("Fire1") && Time.time > nextFire) {
       nextFire = Time.time + fireRate;
       var clone : GameObject = Instantiate (projectile, transform.position, transform.rotation);
        
        
    
    }
 
 
 clone.rigidbody.velocity = transform.TransformDirection(Vector3(0,0,speed));
 
 
 Physics.IgnoreCollision(clone.collider, transform.root.collider);
 
 
 
 }

but when i run the game i get this error: NullReferenceException: Object reference not set to an instance of an object Projectile.Update () (at Assets/GameScripts/Projectile.js:28)

line 28 is this line:

 Physics.IgnoreCollision(clone.collider, transform.root.collider);

What can i do to fix this error?