Im trying to write an xslt function that can apply a 10% discount to the order total when called:

 public void AddVoucherCode(string voucherCode, string orderGuid)
        {
            PercentOffOrderTotalAward percentOffTotal = new PercentOffOrderTotalAward()
            {
                Name = "Voucher Discount",
                PercentOff = 10.00M
            };
            List<PurchaseOrder> orderList = PurchaseOrder.All().ToList();
            PurchaseOrder currentOrder = null;
            foreach (PurchaseOrder order in orderList)
            {
                if (order.OrderGuid.ToString() == orderGuid)
                {
                    currentOrder = order;
                    //update product stock
                    foreach (OrderLine orderLine in order.OrderLines)
                    {
                        string pSku = orderLine.Sku;
                        Product product = Product.All().Where(x => x.Sku == pSku && x.ParentProductId == null).Single();
                        //save the updates quantity for this product 
                        product["InventoryOnHand"].Value = (Convert.ToInt32(product["InventoryOnHand"].Value) - orderLine.Quantity).ToString();
                        //Response.Write("sku: " + product.Sku + " qu: " + product["InventoryOnHand"].Value);
                    }
                    break;
                }
            }
            List<OrderLine> purchaseOrderLines = new List<OrderLine>();
            foreach (OrderLine line in currentOrder.OrderLines)
            {
                purchaseOrderLines.Add(line);
            }
            //percentOffTotal.Apply(currentOrder, purchaseOrderLines);
        }

I get an object reference not set to an instance of object error when i try to apply the discount. is this the correct way to apply?