Unassigned variable is assigned

I am having a strange bug. I have a couple public GameObject variables, and I have set them in the inspector. the enemy variable is fine, but the left_fist variable keeps giving me this error message.

 UnassignedReferenceException: The variable left_fist of 'Player AI' has not been assigned.
     You probably need to assign the left_fist variable of the Player AI script in the inspector.
     Player AI.forward_punch () (at Assets/Player AI.js:29)
     Player AI.Update () (at Assets/Player AI.js:24)

The strange thing is the function still executes, the fist moves. It doesn't come back, that's a separate problem, but it goes out. Does anyone know why I am getting the error?

 var enemy : GameObject;
 var left_fist: GameObject;
 var punching = false;
 var punching_forward = false;
 
 function Update () {
     var distance = Vector3.Distance(enemy.transform.position, transform.position);
     if(enemy.renderer.material.color == Color.red){
         gameObject.renderer.material.color = Color.blue;
     } else if(enemy.renderer.material.color == Color.blue){
         gameObject.renderer.material.color = Color.red;
     } else{
         gameObject.renderer.material.color = Color.green;
     }
     if(Input.GetKeyDown("space")){
         punching = true;
         punching_forward = true;
     }
     forward_punch();
 }
 
 function forward_punch(){
     if(punching){
         var current_fist_position = left_fist.transform.position;
         var distance_from_body = Vector3.Distance(transform.position, current_fist_position);
         if(punching_forward){
             if(distance_from_body < 5){ 
                 var new_position = (current_fist_position + transform.forward);
                 left_fist.transform.Translate(new_position * Time.deltaTime);
             } else { 
                 punching_forward = false;
             }
             Debug.Log(punching_forward);
             Debug.Log(distance_from_body);
             Debug.Log("");
             Debug.Log("");
         } else{
             if(distance_from_body > .5){
                 new_position = (current_fist_position - transform.forward);
                 left_fist.transform.Translate(new_position * Time.deltaTime);
             } else 
                 punching = false;
             }
     }
 }