Thread: reading from xml

Code:

using System.Configuration;
using System;
using System.Collections.Generic;
using System.Text;

namespace ConsoleApplication1
{
    class Program
    {
        static void Main(string[] args)
        {
            String counter = ConfigurationManager.AppSettings["counter"].ToString();
            Console.WriteLine(counter);
        }
    }
}

Code:

<?xml version="1.0" encoding="utf-8"?>
<configuration>
	<appSettings>
		<add key="counter" value="15"/>
	</appSettings>
</configuration>

Unhandled Exception: System.NullReferenceException: Object reference not set to
an instance of an object.

I am a bit tired so I quess there is some stupid mistake I am making here but can you help???