How to avoid an error (XML page cannot be displayed) when exporting GridView records to an Excel file using ASP.Net 4.0 with VS 2010 in C#?

I am creating my first ASP.NET web application.
I try to export a GridView to Excel using the following code but I get the following error once I click on the command button:
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The XML page cannot be displayed
Cannot view XML input using XSL style sheet. Please correct the error and then click the Refresh button, or try again later.
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The operation completed successfully. Error processing resource 'http://localhost:4380/default.aspx'. Line 6, Position 95

   <td>5455</td><td>10/3/2013</td><td>10/3/2013</td><td>New Rejects from current d...
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The C# code for my command button to Export the GridView records to Excel is as follows:

 protected void Button1_Click1(object sender, EventArgs e)
        {
            Response.ClearContent();
            Response.AppendHeader("contenet-disposition", "attachment; filename-Clients.xls");
            Response.ContentType = "application/excel";

            StringWriter stringWriter = new StringWriter();
            HtmlTextWriter htmlTextWriter = new HtmlTextWriter(stringWriter);
            GridView1.RenderControl(htmlTextWriter);
            Response.Write(stringWriter.ToString());
            Response.End();
        }

        public override void VerifyRenderingInServerForm(Control control)
        {

        }