Console app below compiles, but the interface cast fails at run time. Is there an easy way to make this work?

namespace ConsoleApplication1
{
    class Monkey
    {
        public string Shock { get { return "Monkey has been shocked."; } }
    }

    static class MonkeyExtensionToSupportIWombat
    {
        public static string ShockTheMonkey( this Monkey m )
        {
            return m.Shock;
        }
    }

    interface IWombat
    {
        string ShockTheMonkey();
    }

    class Program
    {

        static void Main( string[] args )
        {
            var monkey = new Monkey();
            Console.WriteLine( "Shock the monkey without the interface: {0}", monkey.Shock );
            IWombat wombat = monkey as IWombat;
            Console.WriteLine( "Shock the monkey with the interface: {0}", wombat.ShockTheMonkey() );
            Console.ReadLine();
        }
    }
}