Error in Update,delete,Edit operation in JTable in ASP.net Application, while selection works perfectly

I am having following errors in the ASP.net page . I am using a plugin called jTable( Reference jTable.org). This plugin is used to bind JSON with the HTML UI.

I am using following model

[DataContract]
    public class UserGroup
    {
        public UserGroup()
        {
            GroupId = 0;
        }

        #region Properties

        public short GroupId { get; set; }

        [DataMember]
        public string GroupCode { get; set; }

        [DataMember]
        public char GroupType {get;set;}

        [DataMember]
        public string GroupDescription { get; set; }
        #endregion
    }

Here is the ASPX code

<%@ Page Title="" AutoEventWireup="false" ViewStateMode="Disabled" Language="C#" MasterPageFile="~/Admin/Admin.Master"  CodeBehind="Groups.aspx.cs" 
Inherits="ERP.WebApp.Admin.Groups" %>

<asp:Content ID="Content1" ContentPlaceHolderID="head" runat="server">
    <link href="../Content/themes/redmond/jquery-ui-1.8.16.custom.css" rel="stylesheet" type="text/css" />
    <!-- jTable style file -->

    <link href="/Scripts/jtable/themes/standard/blue/jtable_blue.css" rel="stylesheet" type="text/css" />

    <script src="/Scripts/jquery-ui-1.8.16.custom.min.js" type="text/javascript"></script>
    <script src="/Scripts/modernizr-1.7.min.js" type="text/javascript"></script>
    <script src="/Scripts/jtablesite.js" type="text/javascript"></script>

    <!-- A helper library for JSON serialization -->
    <script type="text/javascript" src="/Scripts/jtable/external/json2.min.js"></script>
    <!-- Core jTable script file -->
    <script type="text/javascript" src="/Scripts/jtable/jquery.jtable.min.js"></script>
    <!-- ASP.NET Web Forms extension for jTable -->
    <script type="text/javascript" src="/Scripts/jtable/extensions/jquery.jtable.aspnetpagemethods.min.js"></script>

</asp:Content>
<asp:Content ID="Content2" ContentPlaceHolderID="ContentPlaceHolder1" runat="server">
<br /><br />
<div style="width:70%;margin:auto;height:500px">
<div id="GroupTableContainer"></div>
</div>
 <script type="text/javascript">

     $(document).ready(function () {

         //Prepare jtable plugin
         $('#GroupTableContainer').jtable({
             title: 'List of User Groups',
             paging: true, //Enables paging
             pageSize: 10, //Actually this is not needed since default value is 10.
             sorting: true, //Enables sorting
             defaultSorting: 'GroupCode ASC', //Optional. Default sorting on first load.
             actions: {
                 listAction: '/Admin/Groups.aspx/GroupsList',
                 createAction: '/Admin/Groups.aspx/CreateGroup',
                 updateAction: '/Admin/Groups.aspx/UpdateGroup',
                 deleteAction: '/Admin/Groups.aspx/DeleteGroup'
             },
             fields: {
                 GroupId: {
                     key: true,
                     create: false,
                     edit: false,
                     list: false                     
                 },
                 GroupCode: {
                     title: 'Group Code',
                     width: '15%'
                 },
                 GroupType: {
                     title: 'Group Type',
                     width: '25%',
                     options: { 'S': 'Student', 'F': 'Faculty', 'A': 'Accounts', 'M': 'Management', 'B': 'Library Staff', 'L': 'Lab Staff', 'E': 'E.R.P. Admins','T':'Training & Placement' }
                 },
                 GroupDescription: {
                     title: 'About the Group',
                     type: 'textarea',
                     width: '50%'
                 }
             }
         });

         //Load Groups list from server
         $('#GroupTableContainer').jtable('load');
     });

</script>
<br /><br />
</asp:Content>

And here is the ASPx.cs code that I am using

[WebMethod(EnableSession = true)]
        public static object GroupsList(int jtStartIndex = 0, int jtPageSize = 10, string jtSorting = null)
        {
            try
            {
                short pTotalRows = 0;
                List<UserGroup> groupList = new GroupsBL().GetAllUserGroups(out pTotalRows,(short)jtStartIndex,(byte) jtPageSize, jtSorting);

                //Return result to jTable
                return new { Result = "OK", Records = groupList, TotalRecordCount = pTotalRows };
            }
            catch (Exception ex)
            {
                return new { Result = "ERROR", Message = ex.Message };
            }
        }

        [WebMethod(EnableSession = true)]
        public static object CreateGroup(UserGroup pUserGroup)
        {
            try
            {
                var addedStudent = new GroupsBL().AddUserGroup(pUserGroup);
                if (addedStudent == true)
                    return new { Result = "OK"};
                else
                    return new { Result = "ERROR", Message = "Group Already Exists" };
            }
            catch (Exception ex)
            {
                return new { Result = "ERROR", Message = ex.Message };
            }
        }

        [WebMethod(EnableSession = true)]
        public static object UpdateGroup(UserGroup pUserGroup)
        {
            try
            {
                var updatedstudent = new GroupsBL().UpdateUserGroup(pUserGroup);
                if (updatedstudent == true)
                    return new { result = "ok"};
                else
                    return new { Result = "ERROR", Message = "Group Already Exists" };
            }
            catch (Exception ex)
            {
                return new { Result = "ERROR", Message = ex.Message };
            }
        }

        [WebMethod(EnableSession = true)]
        public static object DeleteGroup(int pGroupId)
        {
            try
            {
                var deletedStudent = new GroupsBL().DeleteUserGroup((short)pGroupId);
                if(deletedStudent == true)
                return new { Result = "OK" };
                else
                    return new { Result = "ERROR" };
            }
            catch (Exception ex)
            {
                return new { Result = "ERROR", Message = ex.Message };
            }
        }

Selection operation with paging is working fine while any other operation like updation,deletion,creating results in same error. that is shown through Firebug plugin in the screenshot above or as follows.

{"Message":"Invalid web service call, missing value for parameter: \u0027pUserGroup\u0027.","StackTrace":" at System.Web.Script.Services.WebServiceMethodData.CallMethod(Object target, IDictionary2 parameters)\r\n at System.Web.Script.Services.WebServiceMethodData.CallMethodFromRawParams(Object target, IDictionary2 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"}