Can't assign prefab to public transform

I’m simply trying to assign an empty gameobj “EndOfBarrel” to my BulletPrefab’s public transform slot. Unity will not let me do this. I tried putting the bullet pref in the scene and then dragging “EndOfBarrel” to the slot no go either. What is the problem?

bullet script:

using UnityEngine;
using System.Collections;

public class Bullet : MonoBehaviour {
    public Transform endOfBarrel2;


    void Start()

    {

        GameObject.Find ("EndOfBarrel");


    }





    void Update()

   
   
   
    {
        if (transform.position.z > 100)
        {

            transform.position = endOfBarrel2.position;

        }




    }



}

You cannot assign a scene object to an un-instantiated prefab.

Try this it may work, i didn’t test this.

[LIST=1]
[*]void Start()
[*]

[*]    {
[*]

[*]        GameObject.Find ("EndOfBarrel(Clone)");
[*]

[*]

[*]    }
[/LIST]