How to set local currency in AR/Invoice through DI API SAP B1?

Dear Expert,

I am using DI API to add AR/Invoice document, but I can't find how to set local currency for BP with foreign currency.

This my code snip :

        oSO = oCompany.GetBusinessObject(BoObjectTypes.oInvoices)

        oSO.DocDate = dtpPostDate.Value

        oSO.DocDueDate = dtpDueDate.Value

        oSO.TaxDate = dtpDocDate.Value

        oSO.Series = cSeries

        oSO.DocNum = txtNumber.Text

        oSO.DocCurrency = doccur

        oSO.CardCode = txtBP.Text

        oSO.CardName = txtName.Text

        oSO.ContactPersonCode = cntctCode

        oSO.JournalMemo = txtRemarks.Text

        For i = 0 To dgv.RowCount - 2

            oSO.Lines.SetCurrentLine(i)

            oSO.Lines.ItemCode = dgv.Item(0, i).Value

            oSO.Lines.ItemDescription = dgv.Item(2, i).Value

            oSO.Lines.Quantity = dgv.Item(4, i).Value

            oSO.Lines.Price = dgv.Item(5, i).Value

            oSO.Lines.DiscountPercent = dgv.Item(6, i).Value

            oSO.Lines.TaxCode = dgv.Item(7, i).Value

            oSO.Lines.LineTotal = dgv.Item(9, i).Value

            oSO.Lines.Currency = doccur

            oSO.Lines.Add()

        Next

        lRetCode = oSO.Add()

        If lRetCode <> 0 Then

            oCompany.GetLastError(lErrCode, sErrMsg)

            MsgBox(sErrMsg, 16, "Attention")

        Else

            MsgBox("Operation completed successfully.", 64, "Information")

         End If

When trying to Add, system show error message " Enter valid currency code [OINV.DocCur][Line:1], '$' "

I am not lucky after trying for a day , wish enlightenment from the experts here, Thank you.

My machine : SAP B1 9 PL 4

Best Regards,

Muh.Ulfah