Problem with Crystal reports Formula

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C#

.NET

SQL-Server

Visual-Studio

, +

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

<pre lang="c#">
 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 = "{sales.invoice_no}='" +   
                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