IronRuby & Expando object returning null

main.cs

using System;
using System.Dynamic;
using IronRuby;
using IronRuby.Builtins;
using Microsoft.Scripting.Hosting;
using Microsoft.Scripting;

namespace DynamicTest
{
	class MainClass
	{
		public static void Main (string[] args)
		{
			var engine = Ruby.CreateEngine();
			engine.Execute (
@"require 'mscorlib'
require 'DynamicTest.exe'
include DynamicTest
include System
");
			
			var script = engine.CreateScriptSourceFromString (@"
puts ex.foo
");
			dynamic ex = new ExpandoObject ();
			ex.foo = null;
			var scope = engine.CreateScope ();
			scope.SetVariable ("ex", ex);
			script.Execute (scope);
		}
	}
}