jquery, wcf

Below is simple web service called by jquery.
When I comment   var json = $.parseJSON(data); it works ... not sure what's wrong. It does return success when i comment below codes

  var json = $.parseJSON(data);

using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Runtime.Serialization;
using System.ServiceModel;
using System.ServiceModel.Activation;
using System.ServiceModel.Web;
using System.Text;
using System.Web.Script.Serialization;

namespace WcfService
    // NOTE: You can use the "Rename" command on the "Refactor" menu to change the class name "Service" in code, svc and config file together.
    // NOTE: In order to launch WCF Test Client for testing this service, please select Service.svc or Service.svc.cs at the Solution Explorer and start debugging.
    (RequirementsMode = AspNetCompatibilityRequirementsMode.Required)]
    public class Service : IService
        public System.IO.Stream GetSaleTaxJSon(string CompanyID, string CustomerNo, string ShipToCity, string ShipToState, string ShipToZip, string ShipToCountry)
            #region test            
            var j = new { Result = "1" };
            var s = new JavaScriptSerializer();
            string jsonClient = s.Serialize(j);
            WebOperationContext.Current.OutgoingResponse.ContentType = "application/json; charset=utf-8"; //response to the client
            return new MemoryStream(Encoding.UTF8.GetBytes(jsonClient));

<html xmlns="http://www.w3.org/1999/xhtml">
<head runat="server">
    <script src="Scripts/jquery-1.4.1.min.js"></script>
       <script type="text/javascript">
          function GetSaleTax() {
              var city = "EL MONTE";
              var state = "CA";
              var zip = "91731";
              var companyID = "066";
              var country = "USA";
              var customerNo = "337557";
              var GetSaleTaxJSon = "http://localhost:32929/Service.svc?wsdl/WebService/GetSaleTaxJSon";
              $(document).ready(function () {
                      crossDomain: true,
                      cache: false,
                      type: "GET",
                      async: false,
                      dataType: "application/json; charset=utf-8", // "text/plain", //response format
                      contentType: "text/plain", //request format                      
                      url: GetSaleTaxJSon + encodeURIComponent("(" + companyID + "," + customerNo + "," + city + "," + state + "," + zip + "," + country + ")"),
                      success: function (data) {
                          var json = $.parseJSON(data);
                          //$('#resultsSaleTax').html('Sale Tax Web Service<br /><br>' + 'Tax Rate: ' + json.SaleTaxRate + "<br />" + 'Status: ' + json.Status);
                          $('#resultsSaleTax').html('Sale Tax Web Service<br /><br>' + 'Tax Rate: ' +   "<br />" + 'Status: ' + json.Status);
                      error: function (xhr, status, error) {
                          //alert(xhr.responseText + " : error resultsSaleTax");

<body onload="GetSaleTax()">
    <form id="form1" runat="server">
 <div id="resultsSaleTax">

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You have already requested the data to be returned in JSON format by this line  dataType: "application/json; . So the variable data is probably  already JSON and you don't need to do a parseJSON. It's a good idea to inspect your network traffic, browser side, to see what is being returned from the service and how you can use it. I suggest firebug (http://getfirebug.com)  in Firefox, or the develop tools in Chrome (press F12).

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