error when i passed orderid to orderservice

hi
when i passed orderid to order service this apear error
  
page_load
order =orderserv.GetOrderById(this.OrderId);
decimal ordertotal = order.OrderTotal;

lblOrderTotal.Text = ordertotal.ToString();


orderservice

  public Order GetOrderById(int orderId)
        {
            if (orderId == 0)
                return null;

            
            var query = from o in _context.Orders
                        where o.OrderId == orderId
                        select o;
            var order = query.SingleOrDefault();
            return order;
        }