nullreferenceexception object reference not set to an instance of an object

Please Help

 #pragma strict
 public var damage :int;
 
 function Start () {
     damage = damage;
 
 }
 
 function OnTriggerEnter(p : Collider){
     if(p.tag == "Enemy")
     {
     GetComponent(EnemyHealth).HP -=damage;
     Debug.Log("The Enemy got " + damage + " damage");
     }
 
 }

this line is the problem but i don't know how to fix

 GetComponent(EnemyHealth).HP -=damage;