A product must be set before the quantity on an order line

Hi

I am getting the below error:

{A product must be set before the quantity on an order line. (id=1440878712)}

It is based on the following code.

foreach (ic1801_order_detail od in orderLines)
            {
                IOrderLineData orderLine = _sessionOrderLineDataCreate(orderCreated);
                orderLineOrderCurrency = currency;
                string productId = 2000;
                if (odproduct_supplier_reference != null)
                    if (odproduct_supplier_referenceLength  0)
                        productId = odproduct_supplier_reference;
                orderLineProduct = _sessionProductFindByNumber(productId);
                orderLineQuantity = odproduct_quantity;
                orderLineUnitNetPrice = odproduct_price;
                orderLineDescription = odproduct_name;
                IOrderLine orderLineCreated = _sessionOrderLineCreateFromData(orderLine);
            }

My problem is, that the productid is found in the line, so I do not understand why the error occurs:
orderLine.Product = _session.Product.FindByNumber(productId);

When I look into the database it seems like the product has been added to the order.

Any idea, why I have this problem would be appreciated, since I do not understand the cause of this problem. Im using VS2010 SP1

Regards,
Dennis