Error in Crystal Reports

[irfankhan640]

hello Everyone
I am facing problem in Crystal reports in SelectionFormula Line.
When I generate the Report it says A NUMBER IS REQUIRED HERE .
I have tried many things converting to strings to integer but no luck.
Any help will be appreciated. My Code for the Report is below..

pendingorderreportviewer por = new pendingorderreportviewer();
                rptpendingorders rpt = new rptpendingorders();
                DataSet1 ds1 = new DataSet1();
                salesTableAdapter.Fill(ds1.sales);
                sale_detailsTableAdapter.Fill(ds1.sale_details);
                rpt.SetDataSource(ds1);
                por.crystalReportViewer1.ReportSource = rpt;
                por.crystalReportViewer1.SelectionFormula = "{sales.invoice_no}='" + comboBox1.Text + "'";
               por.Show();

invoice_no in the sales table is int.

thanks in Advance