Trying to get an RPC to display the health of a player over their head

 void OnGUI()
     {
         if(photonView.isMine)
         {
             GUI.Label(new Rect(500, 300, 100, 40), "Your hits left: " + playerHealth);
         }
         this.gameObject.GetComponent<PhotonView>().RPC ("ShowHP", PhotonTargets.All, playerHealth);
     }
     
     [RPC]
     void ShowHP(int player_health)
     {
         Vector3 characterPos = Camera.main.WorldToScreenPoint(this.gameObject.transform.position);
         characterPos.y = Screen.height - characterPos.y;
         GUI.Label(new Rect(characterPos.x - 5, characterPos.y - 30, 30, 30), player_health.ToString());
     }

This code should be displaying a gui label above a player's head indicating their HP and everyone should be able to see it. I'm getting a null reference exeception error on the line Vector3 characterPos = Camera.main.WorldToScreenPoint(this.gameObject.transform.position);

and I'm not really sure why. Any ideas?