I am Facing Problem with Formula Selection in Crystal report

Question

Hello eveyrone
I am working on a project in visual studio 2012 with sqlserver backend.
I am creating a Report which is working fine with selecting all the data from the database table, but when i want to select records based on invoice no it generates an error in the Formula.
the error is "A NUMBER IS REQUIRED HERE......"
 
The Code I am Using is

 Pending_order_Report por = new Pending_order_Report();

                rptsales rpt = new rptsales();

                suntechagribusinessDataSet ds1 = new suntechagribusinessDataSet();

                salesTableAdapter.Fill(ds1.sales);

                sale_detailsTableAdapter.Fill(ds1.sale_details);

                rpt.SetDataSource(ds1);

                por.crystalReportViewer1.ReportSource = rpt;

                por.crystalReportViewer1.SelectionFormula = "ToNumber({sales.invoice_no})='" + Convert.ToInt32( listView1.SelectedItems[0].SubItems[1].Text) + "'";

                por.Show();

Pending_Order_report is the form on which i place the Crystal report viewer, invoice number is a column in sale table in the database.
The Above I place on a context menu on the Listview. the Formula Line select listview subitems[1] which is Invoice Number but it generates an error that A NUMBER IS REQUIRED HERE..
Any help will be Appreciated..
Thanks in Advance