Problem accessing script

I have problems accessing function of another script... Where have I made a mistake?

     if(Input.GetMouseButtonDown(0) && isEquiped)    {
         if(name == "Hackmesser")    {
             print("Hack mit Messer");
             
             var target:RaycastHit;
             
             Debug.DrawRay(Camera.main.transform.position, Camera.main.transform.forward * attackRange,Color.red);
             if(Physics.Raycast (Camera.main.transform.position, Camera.main.transform.forward, target, attackRange)) {
                 if(target.collider.tag == "wache")    {
                     print("Hit Wächter");
                     var guard = target.transform.gameObject.GetComponent(Guard);
                     //var guard = FindObjectOfType(Guard);
                     
                     guard.BeDamaged(DamageAmount);
                 }
             }
         }
     }
 

And here is the Guard.js:

 #pragma strict
 
 var health:int = 100;
 
 function Start () {
 
 }
 
 function Update () {
     if(health<0)    {
         Destroy(gameObject);
     }
 }
 
 function BeDamaged(damage:int)    {
     health = health - damage;
 }

Regards Marc

EDIT: This is the error I get: NullReferenceException: Object reference not set to an instance of an object Item.Update () (at Assets/Scripts/Objects/Item.js:35)