What's wrong with this health bar GUI script? (C#)

The script I'm using just doesn't show up at all on the screen when put on First Person Controller... What's wrong with it? I'm not getting any errors.

using UnityEngine; using System.Collections;

public class PlayerHealth : MonoBehaviour { public int maxHealth = 100; public int curHealth = 100;

 // Use this for initialization
 void Start () {
 
 }
 
 // Update is called once per frame
 void Update () {
 
 }
 
 void OnGui() {
     GUI.Box(new Rect(10, 10, Screen.width / 2 / (maxHealth / curHealth), 20), curHealth + "/" + maxHealth);
 }

}