I am getting the following error in my code and I can't seem to understand why. Can anyone help me with it?

This is my current code. The line causing the error is marked in a comment near the end.

var rows : int = 4;
var cols : int = 4;
var totalCards : int = cols * rows;
var matchesNeedToWin : int = totalCards * 0.5; 
var matchesMade : int = 0;

var cardW : int = 100;
var cardH : int = 100;

var aCards : Array; 
var aGrid : Array;

// This Array will store the two cards that the player flipped
var aCardsFlipped : ArrayList; 

// To prevent player from clicking buttons when we don't want him to
var playerCanClick : boolean; 

var playerHasWon : boolean = false;     


class Card extends System.Object {
var isFaceUp : boolean = false;
var isMatched : boolean = false;
var img : String;

    function Card () {
        img = "robot";
    }
}

function Start () {

    var i : int = 0;
    var j : int = 0;

    playerCanClick = true;
    aCards = new Array ();
    aGrid = new Array ();
    aCardsFlipped = new ArrayList ();

    for ( i = 0; i < rows; i++) {
        aGrid [i] = new Array ();

            for (j = 0; j < cols; cols++ ) {
            aGrid [i] [j] = new Card (); // <------ Error over here
        }
    }
}


function Update () {
    Debug.Log("Game Screen has loaded");
}

The error states as follows:

Error BCE0048: Type 'Object' does not support slicing. (BCE0048) (Assembly-UnityScript)