A/P Down Payment Request error "Base document card and target document card do not match" C#

When Add Document get this error "Base document card and target document card do not match"

SAPbobsCOM.Documents oPDPMRequest = (SAPbobsCOM.Documents)fmCo.GetBusinessObject(SAPbobsCOM.BoObjectTypes.oPurchaseDownPayments);
if (oPDPMRequest.GetByKey(int.Parse(FldVal))) 

  SAPbobsCOM.Documents oPDPMInvoice = (SAPbobsCOM.Documents)fmCo.GetBusinessObject(SAPbobsCOM.BoObjectTypes.oPurchaseDownPayments);
                    
  //Set Down Payment Header Values
  oPDPMInvoice.CardCode = oPDPMRequest.CardCode;
  oPDPMInvoice.DocDueDate = DateTime.Now;
  oPDPMInvoice.DownPaymentPercentage = oPDPMRequest.DownPaymentPercentage;
  oPDPMInvoice.VatPercent = oPDPMRequest.VatPercent;
  oPDPMInvoice.DiscountPercent = 0;
  oPDPMInvoice.DocTotal = oPDPMRequest.DocTotal;
  oPDPMInvoice.DocRate = oPDPMRequest.DocRate;
  oPDPMInvoice.GroupNumber = oPDPMRequest.GroupNumber;
  oPDPMInvoice.DeferredTax = oPDPMInvoice.DeferredTax;

 //Set Down Payment Type
 oPDPMInvoice.DownPaymentType = SAPbobsCOM.DownPaymentTypeEnum.dptInvoice;

 //Set Down Payment Line Values              
  oPDPMInvoice.Lines.BaseType = 22;
  oPDPMInvoice.Lines.BaseEntry = oPDPMRequest.DocEntry;

   if (oPDPMInvoice.Add() != 0)
    {
         Application.SBO_Application.MessageBox(fmCo.GetLastErrorDescription());
         return;
      }
    else
      {
        Application.SBO_Application.MessageBox("Purchase Invoice [" + fmCo.GetNewObjectKey() + "] created!");
      }
    }