Error in adding production order

I am using the object of PO for adding new PO. The code is as follows -

objPO = objCompany.GetBusinessObject(BoObjectTypes.oProductionOrders)

                                objPO.ItemNo = txtBOMCode.Value

                                objPO.Warehouse = "SFGMF"

                                strQuery = "SELECT Series FROM NNM1 WHERE BeginStr='SPO' AND LOCKED='N'" _

                                & " AND Indicator IN (SELECT Indicator FROM NNM1 WHERE Series=" & intSeries & ")"

                                intSeries = GetDataByScaler(strQuery, "AddPO")

                                objPO.Series = intSeries

                                objPO.PlannedQuantity = Val(txtQty.Value)

                                dblPlannedQty = Val(txtQty.Value)

                                objPO.PostingDate = dtDate

                                objPO.DueDate = dtDate.AddDays(objItem.LeadTime)

                                objPO.Remarks = "Based on PO " & objSubWordOrd.DataSources.UserDataSources.Item("Series").Value & "-" & Replace(objSubWordOrd.DataSources.UserDataSources.Item("DocNum").Value.ToString, ".000000", "")

                                lngStatus = objPO.Add

The variable lngStatus returns value -10. When I viewed error using objCompany.GetLastErrorDescription it gave error as ,

10001385 - Enter valid number; document number already exists