nullrefernceexception

i am getting a error when i use the following code when i try to add a velocity to the object and i am not sure why i am getting it as i am using the same code in a different script and not getting any errors

 public Rigidbody Rocks;
 private float NextShoot;
 private Rigidbody CloneRocks;
 
 void Start () {
 NextShoot = Time.time +1;
 }
 
 
 void Update () {
 if(Time.time > NextShoot)
     {
         NextShoot =  Time.time + 10.0f;    
         CloneRocks = Instantiate(Rocks,transform.position ,transform.rotation) as Rigidbody;
         
         this.CloneRocks.rigidbody.velocity = transform.TransformDirection (Vector3.forward * 10);
         
     
     }
 }