Invoice DocNum

I have managed to create an AR Invoice record using the following code:

private SAPbobsCOM.Documents oInvoice;

oInvoice = ( ( SAPbobsCOM.Documents )( oCompany.GetBusinessObject( SAPbobsCOM.BoObjectTypes.oInvoices ) ) );

int lRetCode;

oInvoice.CardCode = "979";

oInvoice.DocDate = DateTime.Parse("18.07.2007");

oInvoice.DocDueDate = DateTime.Parse("18.07.2007");

oInvoice.TaxDate = DateTime.Parse("18.07.2007");

oInvoice.Lines.ItemCode = "12345";

oInvoice.Lines.Quantity = 2;

oInvoice.Lines.Price = 10.00;

lRetCode = oInvoice.Add();

The problem is that I want to give the invoices my own DocNum. I tried doing this by using this code:

oInvoice.Series = -1;

oInvoice.DocNum = 9999;

but it doesn't create any record of the invoice. Can anyone help?