A question related to the explosion after creation of debris

Hi mates!.

The fact is that I've got an object which disappears after clicking on it thanks to the first script. Then I apply another (the second) script which makes possible that a debris fall down from the disappearing position and I wanted to make, between these phases, an explosion or some particles effect.

So I created in the second script (which creates the debris) an instance of the particles effect at the same time (I suppose it) I create the debris.

But there's no effect. It doesn't work.

I receive a message through teh console like this: "UnassignedReferenceException: The variable explosion of 'newspam' has not been assigned. You probably need to assign the explosion variable of the newspam script in the inspector."

It must be something very simple.

Here's my code:

 // Instantiate a rigidbody then set the velocity
 var projectile : Rigidbody;
 var explosion : Transform;
 function Update () {
 // Ctrl was pressed, launch a projectile
 if (Input.GetButtonDown("Fire1")) {
 // Instantiate the projectile at the position and rotation of this transform
 var clone : Rigidbody;
 var humo : Transform;
 clone = Instantiate(projectile, transform.position, transform.rotation);
 // Give the cloned object an initial velocity along the current
 // object's Z axis
 clone.velocity = transform.TransformDirection (Vector3.forward * 2);
 humo = Instantiate(explosion, transform.position, transform.rotation);
 }
 }

Thanks in adavance.