Why is this giving me a null refrence exeption?

I have a ray that shoots out of this collider and when it hits its suppose to detect that it hit an AI but not the player. So I tried this code:

 if(hit.rigidbody.tag == "AI")

But it keeps giving me a null refrence exeption. But if I try it like this:

 if(hit.rigidbody == True)

It hits the AI and the player. Why is it not finding the tag? Is there a better way to go about this?

Edit:

This is the code thats on the collider

 #pragma strict
 var carWaiting   : boolean = false;
 private var stop : String  = "Intersection";
 private var go   : String  = "Go";
 function Start ()
 {
     // This makes sure the colliders tag starts out as stop
     gameObject.tag = stop;
 }
 
 function Update () 
 {
     var back = transform.TransformDirection (Vector3.back);
      var hit : RaycastHit;
         
     if(Physics.Raycast(transform.position, back, hit, 100))
     {
      // The ray detects it hits something here
         if(hit.collider.gameObject.tag == "AI")
         {
                    //but doesn't detect the AI here
             print("I see a computer car");
             carWaiting = true;
         }
     }
 }