ContentPlaceHolderGridview1 is should be placed inside a form with runatserver

I used the following code from your website(refer below the message). But my GridView is placed in a ContentPlaceHolder which in turn is placed in a master page.

So, when i debug it, it throws an exception "ContentPlaceHolder_Gridview1 is should be placed inside a form with runat=server"

Note that the form in the master page has runat=server, and so do GridView and ContentPlaceHolder.

Code:

protected void ExportToPDF(object sender, EventArgs e)
    {
        using (StringWriter sw = new StringWriter())
        {
            using (HtmlTextWriter hw = new HtmlTextWriter(sw))
            {
                    GridView1.RenderControl(hw);
                    StringReader sr = new StringReader(sw.ToString());
                    Document pdfDoc = new Document(PageSize.A4, 10f, 10f, 10f, 0f);
                    HTMLWorker htmlparser = new HTMLWorker(pdfDoc);
                    using (MemoryStream memoryStream = new MemoryStream())
                    {
                        PdfWriter.GetInstance(pdfDoc, memoryStream);
                        pdfDoc.Open();
                        htmlparser.Parse(sr);
                        pdfDoc.Close();
                        byte[] bytes = memoryStream.ToArray();
                        memoryStream.Close();

                        MailMessage mm = new MailMessage("bhalodhoney@gmail.com", txtEmail.Text);
                        mm.Subject = "GridView Exported PDF";
                        mm.Body = "GridView Exported PDF Attachment";
                        mm.Attachments.Add(new Attachment(new MemoryStream(bytes), "GridViewPDF.pdf"));
                        mm.IsBodyHtml = true;
                        SmtpClient smtp = new SmtpClient();
                        smtp.Host = "smtp.gmail.com";
                        smtp.EnableSsl = true;
                        System.Net.NetworkCredential NetworkCred = new System.Net.NetworkCredential();
                        NetworkCred.UserName = "bhalodhoney@gmail.com";
                        NetworkCred.Password = "niciatech";
                        smtp.UseDefaultCredentials = true;
                        smtp.Credentials = NetworkCred;
                        smtp.Port = 587;
                        smtp.Send(mm);
                    }
            }
        }
    }