ASP.Net Error: Cannot convert null to a value type

{"Message":"Cannot convert null to a value type.","StackTrace":"   at System.Web.Script.Serialization.ObjectConverter.ConvertObjectToTypeMain(Object o, Type type, JavaScriptSerializer serializer, Boolean throwOnError, Object\u0026 convertedObject)\r\n   at System.Web.Script.Services.WebServiceMethodData.StrongTypeParameters(IDictionary`2 rawParams)\r\n   at System.Web.Script.Services.WebServiceMethodData.CallMethodFromRawParams(Object target, IDictionary`2 parameters)\r\n   at System.Web.Script.Services.RestHandler.InvokeMethod(HttpContext context, WebServiceMethodData methodData, IDictionary`2 rawParams)\r\n   at System.Web.Script.Services.RestHandler.ExecuteWebServiceCall(HttpContext context, WebServiceMethodData methodData)","ExceptionType":"System.InvalidOperationException"}

        [WebMethod]
        public static string Insert(int id, string comment)
        {
            string constr = ConfigurationManager.ConnectionStrings["DB"].ConnectionString;
            using (SqlConnection con = new SqlConnection(constr))
            {
                using (SqlCommand cmd = new SqlCommand())
                {
                    cmd.CommandText = "INSERT INTO PostComment2 (Email,CommentID,Comments) VALUES (@Email,@CommentID,@Comments)";
                    cmd.CommandType = CommandType.Text;
                    cmd.Connection = con;
                    cmd.Parameters.AddWithValue("@CommentID", id);
                    cmd.Parameters.AddWithValue("@Email", HttpContext.Current.User.Identity.Name);
                //    cmd.Parameters.AddWithValue("@SendDate", name);
                    cmd.Parameters.AddWithValue("@Comments", comment);
                    con.Open();
                    cmd.ExecuteNonQuery();
                    con.Close();
                }
            }
            return "";
        }

 <script type="text/javascript">
    $(function () {
        $('[id*=btncomtpost]').on('click', function () {
            var row = $(this).closest('tr').closest('table').parent();
            var comment = $(this).closest('tr').find('[id*=txtcommentpost2]').val();
            var id = $(this).closest('tr').closest('table').parent().find('[id*=lblCommentID]').html();
            var obj = {};
            obj.id = parseInt(id);
            obj.comment = comment;
            $.ajax({
                type: "POST",
                 url: "/Home.aspx/Insert",
                data: JSON.stringify(obj),
                contentType: "application/json; charset=utf-8",
                dataType: "json",
                success: function (response) {
                    $(row).find('tr').eq(0).before('<tr><td>' + comment + '</td></tr>');
                    $(row).find('tr').find('[id*=txtcommentpost2]').val('');
                },
                failure: function (response) {
                    alert(response.responseText);
                },
                error: function (response) {
                    alert(response.responseText);
                }
            });
 
            return false;
        });
    });
</script>