I have a little problem with instantiate have a code as that ;

    var prefab: Transform;


function Update () {
    if (Input.GetMouseButton(0)) {
        var ray: Ray = Camera.main.ScreenPointToRay(Input.mousePosition);
        var hit: RaycastHit;

        if (Physics.Raycast(ray, hit)) {
            Instantiate(prefab, hit.point, Quaternion.identity);
        } 
    }
}

I have a Cube as prefab. When i play and clicked to any area i take this error.

NullReferenceException: The prefab you want to instantiate is null. UnityEngine.Object.Instantiate (UnityEngine.Object original, Vector3 position, Quaternion rotation) NewBehaviourScript 1.Update () (at Assets\NewBehaviourScript 1.js:12)

How can i insert my prefab where my clicked ?

Can you help me ?