Is it perhaps not passing the Invoice PK to the subreport? In the main report, right click on the cell/textbox that contains the subreport and go to Properties. In Parameters, there should be something like

Parameter NameParameter Value

@invoicePK=Fields!InvoicePK.Value

This passes the invoice parameter from whatever field it occupies on the main report to the subreport. From the verbiage of the error I'm not sure this is the problem, but at least it's one thing to check.