NullReferenceException: Object reference not set to an instance of an object

i made an animation of my weapon swinging and it was supposed to play whenever i click the mouse button. now it said "NullReferenceException: Object reference not set to an instance of an object MeleeSystem.Update () (at Assets/MeleeSystem.js:11)" ???? i named my weapon "Hammer". this is my code:

 #pragma strict
 
 var thedamage : int = 50;
 var Distance : float;
 var MaxDistance : float = 1.5;
 var Hammer : Transform;
 
 function Update () 
 {   if (Input.GetButtonDown("Fire1"));
     {
         Hammer.animation.Play("attack");
         var hit : RaycastHit;
         if (Physics.Raycast (transform.position, transform.TransformDirection(Vector3.forward), hit))
         {
             Distance = hit.distance;
             if (Distance < MaxDistance)
             {    
                 hit.transform.SendMessage("ApplyDamage", thedamage, SendMessageOptions.DontRequireReceiver);
             }
         }
     }
 }