ASP.net convert gridview data to pdf

protected void Button4_Click(object sender, EventArgs e)
{
    Response.ContentType = "application/pdf";
    Response.AddHeader("content-disposition","attachment;filename=GridViewExport.pdf");
    Response.Cache.SetCacheability(HttpCacheability.NoCache);
    StringWriter sw = new StringWriter();
    HtmlTextWriter hw = new HtmlTextWriter(sw);
    GridView1.AllowPaging = false;
    GridView1.DataBind();
    GridView1.RenderControl(hw);
    StringReader sr = new StringReader(sw.ToString());
    Document pdfDoc = new Document(PageSize.A4, 10f, 10f, 10f, 0f);
    HTMLWorker htmlparser = new HTMLWorker(pdfDoc);
    PdfWriter.GetInstance(pdfDoc, Response.OutputStream);
    pdfDoc.Open();
    htmlparser.Parse(sr);//this is the error line
    pdfDoc.Close();
    Response.Write(pdfDoc);
    Response.End();  
}