random sprite javascript

I try to generate random sprite, but i have "Array index is out of range" I don't understand what i am doing wrong. Here is my code:

 #pragma strict
 
 var sprites : Sprite[];
 var resourceName : String = "Sprites/spriteLetters";
 var currentSprite : int = -1;
 
 function Start () {
     if(resourceName != "") {
         sprites = Resources.LoadAll.<Sprite>(resourceName);
         if(currentSprite == -1) {
             currentSprite = Random.Range(0, sprites.Length);
         }
         else if(currentSprite > sprites.Length) {
             currentSprite = sprites.Length - 1;
         }
         
         GetComponent(SpriteRenderer).sprite = sprites[currentSprite];
 
     }
 }