OnGui function - Array is out of index question

Hi fellow Uniters, anyone have any idea why I keep getting an "Array is out of index" errors on the following line:

GUI.DrawTexture(playerHealthRect, lifeBarTexture[i], ScaleMode.ScaleToFit, true, 0.0f);

I got this code from another post. Thanks for the help. The complete code is below:

// LifeBar.cs - script to handle player and enemy lifebar

using UnityEngine;
using System.Collections;

public class LifeBar : MonoBehaviour
{
public Texture2D[] lifeBarTexture;
public int playerHealth = 10;
public int playerHealthTextureSize = 30;

 void OnGUI ()
 {
     Rect playerHealthRect = new Rect(10, 5, playerHealthTextureSize, playerHealthTextureSize);
     for (int i = 0; i < playerHealth; i++)
     {
         GUI.DrawTexture(playerHealthRect, lifeBarTexture[i], ScaleMode.ScaleToFit, true, 0.0f);
         playerHealthRect.x += playerHealthTextureSize + 10;
     }
 }
 
 void OnCollisionEnter (Collision collision)
 {
     if (collision.collider.tag == "enemy")
     {
         if (playerHealth -1 >= 0)
             lifeBarTexture[playerHealth -1] = null;
         playerHealth--;
     }
 }

}