Exception on WebService Method Callback

VS 2k13

.NET 4.5

At the time of adding web service reference, I clicked on advanced and opted in to generate async operations as shown here :

http://techybit.com/wp-content/uploads/2011/08/image3.png



I am getting an exception on line # 29

An exception of type 'System.ArgumentException' occurred in mscorlib.dll but was not handled in user code

Additional information: Async End called with an IAsyncResult from a different Begin method.

Code :

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using ConsoleApplication2.ServiceReference1;

namespace ConsoleApplication2
{
    class Program
    {
        //address : http://wsf.cdyne.com/WeatherWS/Weather.asmx?WSDL

        static void Main(string[] args)
        {
            string zip = "60601";
            ServiceReference1.WeatherSoapClient client = new WeatherSoapClient("WeatherSoap");
            AsyncCallback cb = new AsyncCallback(Program.GetWeatherByZipCodeCallback);
            IAsyncResult ar = client.BeginGetCityWeatherByZIP(zip, cb, client);
            Console.WriteLine("Getting Data ...");
            while (!ar.IsCompleted);
            Console.ReadKey();
        }

        public static void GetWeatherByZipCodeCallback(IAsyncResult ar)
        {
            Console.Clear();
            WeatherSoapClient client = (WeatherSoapClient)ar.AsyncState;
            ForecastReturn result = client.EndGetCityForecastByZIP(ar);    // here 
            Console.WriteLine(result.City);
        }
    }
}

reference : MSDN man page