خطای the tag was not recognized در xml

 int count = int.Parse(GridView1.Rows[0].Cells[0].Text);
        Response.Clear();
        Response.ContentType = "text/xml";
        using (XmlWriter writer = XmlWriter.Create(Server.MapPath("sitemap.xml")))
        {
            DAL.setting lo = DAL.setting.Select_Key(count);
            writer.WriteStartDocument();
            writer.WriteStartElement("urlset");
             

            writer.WriteElementString("title", lo.name_web);
            SqlConnection cnn = new SqlConnection(Utility.Connection_String());
            //SqlConnection cnn = new SqlConnection(Connection.GetConnect());
            SqlCommand cmd = new SqlCommand();
            cmd.Connection = cnn;
            cmd.CommandText = "Select * From post WHERE state='False' order by [ID_post] desc";
            cmd.CommandType = CommandType.Text;
            SqlDataAdapter dap = new SqlDataAdapter(cmd);
            DataSet ds = new DataSet();
            dap.Fill(ds, "ds");
            foreach (DataRow dr in ds.Tables[0].Rows)
            {

                string Loc = "http://" + lo.url_web + "/news/" + dr["ID_post"].ToString() + "/" + dr["url_post"].ToString() + ".html";
                writer.WriteStartElement("url");
                writer.WriteElementString("loc", Loc);
                writer.WriteElementString("changefreq", changefreq);
                writer.WriteElementString("priority", priority);
                writer.WriteEndElement();
            }




            writer.WriteEndElement();

            {
                writer.Flush();
                Response.Redirect("sitemap.xml");
            }
        }

سلام به شما

اگر کد xml شما خیلی طولانی نیست آن را هم در سوال بیاورید