IronRuby: Coercion error with String and MutableString

main.cs

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

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 (@"
d = DynamicTestClass.new
d.foo 'test'
");
			script.Execute ();
		}
	}
	
	public class DynamicTestClass : DynamicObject
	{
		public override bool TryGetMember (GetMemberBinder binder, out object result)
		{
			result = null;
			return true;
		}
		
		public override bool TryInvokeMember (InvokeMemberBinder binder, object[] args, out object result)
		{
			result = null;
			return true;
		}
	}
}