Create a partial Goods receipt po with serial numbers DI

I am trying to create a partial goods receipt PO based on a purchase order. my code is below

I can do a full po receipt. But trying to do a partial gives me the following error.

- 4014 cannot add row without complete selection of batch/serial numbers.

I debugged the code and I am adding all serial numbers necessary - I think its something to do with setting the quantity but I cannot figure this out.

any help is much appreciated.

SAPbobsCOM.Documents documents = (SAPbobsCOM.Documents)oCompany.GetBusinessObject(SAPbobsCOM.BoObjectTypes.oPurchaseDeliveryNotes);

                documents.CardCode = sPOVendorCode.Trim();

                documents.HandWritten = SAPbobsCOM.BoYesNoEnum.tNO;               

                documents.DocDate = Convert.ToDateTime( oEd.String);                

                documents.DocDueDate = Convert.ToDateTime(oEd.String);

                documents.Lines.SetCurrentLine(0); // there will be on,y one               

                documents.Lines.BaseEntry = Convert.ToInt32(lPODocEntry);

                documents.Lines.BaseLine = 0;

                documents.Lines.BaseType = 22;

                documents.Lines.Quantity = Convert.ToInt32(dListQuantity);

                documents.Lines.SerialNumbers.Quantity = Convert.ToInt32(dListQuantity);               

                documents.Lines.RequiredQuantity = Convert.ToInt32(dListQuantity);

                Rs.DoQuery(sSql);

                if (!(Rs.EoF && Rs.BoF))

                {

                    int i = 0;

                    while (!(Rs.EoF))

                    {                       

                        documents.Lines.SerialNumbers.SetCurrentLine(i);

                        documents.Lines.SerialNumbers.InternalSerialNumber = sSerNo;                     

                        documents.Lines.SerialNumbers.Add();

                        Rs.MoveNext();

                        i++;

                    }

                }               

                documents.Lines.Add();

                int resp = documents.Add();