Prefab and certain script makes unity crash.

When i make a prefab and add this script:

 #pragma strict
 
 public var damage: float = 5;
 public var stuckArrowPosition: Vector3;
 public var stuckArrowRotation: Quaternion;
 public var hit: boolean = false;
 public var rb = GetComponent.<Rigidbody2D>();
 public var tagNotToKill: String;
 
 function OnCollisionEnter2D ( col: Collision2D ) {
     if (col.gameObject != null && hit == false && col.gameObject.tag != tagNotToKill) {
         Debug.Log(col);
         col.gameObject.SendMessage("ApplyDamage", damage);
     } 
     else 
     {
         transform.parent = col.transform;
         rb.isKinematic = true;
         rb.fixedAngle = true;
         hit = true;
         Destroy(this.gameObject, 15);
     }
 }

Unity crashes as soon as I add it. If i add it to a game object which isn't a prefab, it doesn't crash and if I use a different script it doesn't crash, even on a prefab. I've tried creating the script from the menu inside the prefab, and dragging in one i made before, and none of them work.