sortOn() crash

// ADD item to cart
        		_root.productDisplay.bottom.addToCart.onRelease = function() {
                    for(z in _global.cart) {
            			if(_global.cart[z].id == _root.productItem[idNo].attributes.id) {
            				_global.cart[z].Quantity++;
            				var foundit = true;
            				break;
            			}
                    }
                    if(!foundit) {
                        _global.cart.push({
                            id:_root.productItem[idNo].attributes.id, 
                            Manufacturer:item.mName, 
                            Product:item.pName, 
                            Price:item.ourPrice, 
                            Weight:Number(_root.productItem[idNo].attributes.weight),
                            unitSize:_root.productItem[idNo].attributes.pedal_units,
                            Quantity:Number(1)
                            });
                    }

// REMOVE item from cart
_root.cartDetails.removeItemButton.onRelease = function() {
    for(z in _global.cart) {
        if(_global.cart[z].id == theSelectedItem.id) {
            delete _global.cart[z];
            break;
        }
    }
	displayCartDetails();
	refreshShoppingCart();
};

// Sort the array
    _global.cart.sortOn("unitSize", Array.NUMERIC); // (hangs flash.)