Invalid object passed in : or expected

I am getting this error when i submit my form.

I get only when there is some ' or some character in it 

<script type="text/javascript">
        $(document).ready(function () {
            $('.savedata').on("click", function () {
                var converter = new showdown.Converter(),
                text = $('#<%=messageBody.ClientID %>').val(),
                html = converter.makeHtml(text);
                if ($("#IsSchedule").is(":checked")) {
                    var isschedule = parseInt(1);

                }
                else {
                    var isschedule = parseInt(0);
                }

                $.ajax({
                    type: "POST",
                    url: "/Profile/StartDiscussion.aspx/updateData",
                    data: '{"DTitle":"' + $.trim($('#<%=TextBox1.ClientID %>').val()) + '","DMdContent":"/' + $('#<%=messageBody.ClientID %>').val() + '","DInvitedPeople":"' + $.trim($('#<%=InvitePeople.ClientID %>').val()) + '","DIsScheduled":"'+ isschedule + '","DScheduledDate":"' + $.trim($('#datetimescheduled').val()) + '","DCategory":"' + $.trim($("[id*='OptionsSelected'] :selected").text()) + '","DHtmlContent":"/' + html +'"}',
                    contentType: "application/json; charset=utf-8",
                    dataType: "json",
                    success: function (data) {
                        var obj = data.d;
                        if (obj == 'true') {
                           $('.postsubmit').css({ 'display': 'block' });
                            $('.postsubmit').delay(6000).fadeOut();

                        }
                    },
                    error: OnError

                });
            });
        });
        function OnError(xhr, errorType, exception) {
            var responseText;
            $("#dialog").html("");
            try {
                responseText = jQuery.parseJSON(xhr.responseText);
                $("#dialog").append("<div><b>" + errorType + " " + exception + "</b></div>");
                $("#dialog").append("<div><u>Exception</u>:<br /><br />" + responseText.ExceptionType + "</div>");
                $("#dialog").append("<div><u>StackTrace</u>:<br /><br />" + responseText.StackTrace + "</div>");
                $("#dialog").append("<div><u>Message</u>:<br /><br />" + responseText.Message + "</div>");
            } catch (e) {
                responseText = xhr.responseText;
                $("#dialog").html(responseText);
            }
            $("#dialog").dialog({
                title: "jQuery Exception Details",
                width: 700,
                buttons: {
                    Close: function () {
                        $(this).dialog('close');
                    }
                }
            });
        }

        $(document).ajaxStart(function () {
            NProgress.start();
        });
        $(document).ajaxStop(function () {
            NProgress.done();

        });
    </script>

 [WebMethod]
    [ScriptMethod]
    public static string updateData(string DTitle, string DMdContent, string DInvitedPeople, int DIsScheduled, string DScheduledDate, string DCategory, string DHtmlContent)
    {
     
        String msg = String.Empty;
        String decoded = Password.Decrypt(HttpContext.Current.Request.Cookies["UserEmail"].Value);
        string conString = ConfigurationManager.ConnectionStrings["GitaAdmin"].ConnectionString;
        using (SqlConnection con = new SqlConnection(conString))
        {
            using (SqlCommand cmd = new SqlCommand("INSERT into DiscussionTable(DTitle, DMdContent, DActive , DApproved , DEmailSent , DStartedBy , DInvitedPeople , DIsScheduled , DScheduledDate, DCategory,DHtmlContent)"
               + "Values(@DTitle  ,@DMdContent , @DActive , @DApproved , @DEmailSent , @DStartedBy , @DInvitedPeople , @DIsScheduled , @DScheduledDate,@DCategory,@DHtmlContent)"))
            {
                cmd.Connection = con;
                cmd.Parameters.AddWithValue("@DTitle", DTitle);
                cmd.Parameters.AddWithValue("@DMdContent", DMdContent);
                cmd.Parameters.AddWithValue("@DActive", 0);
                cmd.Parameters.AddWithValue("@DApproved", 0);
                cmd.Parameters.AddWithValue("@DEmailSent", 0);
                cmd.Parameters.AddWithValue("@dstartedby", decoded);
                cmd.Parameters.AddWithValue("@DInvitedPeople", DInvitedPeople);
                cmd.Parameters.AddWithValue("@DIsScheduled", DIsScheduled);
                cmd.Parameters.AddWithValue("@DScheduledDate", DScheduledDate);
                cmd.Parameters.AddWithValue("@DCategory", DCategory);
                cmd.Parameters.AddWithValue("@DHtmlContent", DHtmlContent);
                con.Open();
                int rowsAffected = cmd.ExecuteNonQuery();
                con.Close();
                if (rowsAffected == 1)
                {
                    msg = "true";
                }
                else
                {
                    msg = "false";
                }

            }
        }
        return msg;
    }