ArgumentOutOfRange Exception

Hi guys, I bought the book Unity Game Development by Example Beginner's Guide. And in chapter 5, Making a robot repair game, i keep getting this error

ArgumentOutOfRangeException: Index is less than 0 or more than or equal to the list count. The error is from 12th line inside Start() Function just below the Random.Range code

I dont know how to solve this issue, Please help...

Here is the code..

 var cols:int = 4; // the number of columns in the card grid
 var rows:int = 4; // the number of rows in the card grid
 var totalCards:int = cols*rows; // I think the answer is 16, but I was never good with numbers.
 var matchesNeededToWin:int = totalCards * 0.5; // If there are 16 cards, the player needs to find 8 matches to clear the board
 var matchesMade:int = 0; // At the outset, the player has not made any matches
 var cardW:int = 100; // Each card's width and height is 100 pixels
 var cardH:int = 100;
 var aCards:Array; // We'll store all the cards we create in this Array
 var aGrid:Array; // This array will keep track of the shuffled, dealt cards
 var aCardsFlipped:ArrayList; // This array will store the two cards that the player flips over
 var playerCanClick:boolean; // We'll use this flag to prevent the player from clicking buttons when we don't want him to
 var playerHasWon:boolean = false; // Store whether or not the player has won. This should probably start out false :)
 
 function Start()
 {
     playerCanClick = true; // We should let the player play,    don't you think?
     // Initialize the arrays as empty lists:
     aCards = new Array();
     aGrid = new Array();
     aCardsFlipped = new ArrayList();
     for(var i=0; i<rows; i++)
     {
         aGrid[i] = new Array(); // Create a new, empty array at index i
         for(var j=0; j<cols; j++)
         {
             var someNum:int = Random.Range(0,aCards.length);
             aGrid[i][j] = aCards[someNum];
             aCards.RemoveAt(someNum);
         }
     }
     BuildDeck();
 }
 
 class Card extends System.Object
 {
     var isFaceUp:boolean = false;
     var isMatched:boolean = false;
     var img:String;
     function Card(img:String)
     {
         this.img = img;
     }
 }
 
 function OnGUI ()
 {
     GUILayout.BeginArea (Rect (0,0,Screen.width,Screen.height));
     BuildGrid();
     GUILayout.EndArea();
     print("building grid!");
     
 }
 
 function BuildGrid()
 {
     GUILayout.BeginVertical();
     GUILayout.FlexibleSpace();
     for(var i=0; i<rows; i++)
     {
         GUILayout.BeginHorizontal();
         GUILayout.FlexibleSpace();
         for(var j=0; j<cols; j++)
         {
             var card:Object = aGrid[i][j];
             if(GUILayout.Button(Resources.Load(card.img),GUILayout.Width(cardW)))
             {
                 Debug.Log(card.img);
             }
         }
         GUILayout.FlexibleSpace();
         GUILayout.EndHorizontal();
     }
     GUILayout.FlexibleSpace();
     GUILayout.EndVertical();
 }
 
 function BuildDeck() {
     
     var totalRobots:int = 4; // we've got four robots to work with
     var card:Object; // this stores a reference to a card
     for(i=0; i<totalRobots; i++)
     {
         var aRobotParts : Array = ["Head", "Arm", "Leg"];
         for(j=0; j<2; j++)
         {
             var someNum:int = Random.Range(0, aRobotParts.length);
             var theMissingPart:String = aRobotParts[someNum];
             aRobotParts.RemoveAt(someNum);
             card = new Card("robot" + (i+1) + "Missing" + theMissingPart);
             aCards.Add(card);
             card = new Card("robot" + (i+1) + theMissingPart);
             aCards.Add(card);
         }
     }
 }