FPS Tutorial part 2 - please help

I'm following the FPS tut, and i have a problem: when i shoot my rocket launcher, the rocket is supposed to explode when it hits something. instead, it just bounces off and flies around for 2-4 seconds before disappearing. there's also an error that says

NullReferenceException: The prefab you want to instantiate is null. UnityEngine.Object.Instantiate (UnityEngine.Object original, Vector3 position, Quaternion rotation) Rocket.OnCollisionEnter (UnityEngine.Collision collision) (at Assets\WeaponScripts\Rocket.js:18)

this is the Rocket.js Script:

 // The reference to the explosion prefab
var explosion : GameObject;
var timeOut = 3.0;

// Kill the rocket after a while automatically
function Start () 
    Invoke("Kill", timeOut);

function OnCollisionEnter (collision : Collision) {
    // Instantiate explosion at the impact point and rotate the explosion
    // so that the y-axis faces along the surface normal
    var contact : ContactPoint = collision.contacts[0];
    var rotation = Quaternion.FromToRotation(Vector3.up, contact.normal);
    Instantiate (explosion, contact.point, rotation);
    // And kill our selves
    Kill ();    

function Kill () {
    // Stop emitting particles in any children
    var emitter : ParticleEmitter= GetComponentInChildren(ParticleEmitter);
    if (emitter)
        emitter.emit = false;

    // Detach children - We do this to detach the trail rendererer which should be set up to auto destruct

    // Destroy the projectile

@script RequireComponent (Rigidbody)

when you click the rocket prefab in your project folder in the rocket script make sure that the explosion var is set to something......... there should be a explosion prefab in your standard assets drag this prefab into the empty var slot.....