Subclassing NSMutableAttributedString causes an error "[CustomNSAttributedString initWithAttributedString:]: unrecognized selector sent to instance 0x4418b0"

Main.cs

using System;
using MonoMac.Foundation;
using MonoMac.CoreText;
using MonoMac.AppKit;

namespace SubclassingTest
{
	public class Program
	{
		static void Main (string[] args)
		{
			NSApplication.Init ();
			var attrs = new CTStringAttributes () {
				Font = new CTFont ("HoeflerText-Regular", 24.0f),
				LigatureFormation = CTLigatureFormation.All,
				KerningAdjustment = 10,
				UnderlineStyle = CTUnderlineStyle.Single,
				StrokeWidth = 5
			};

			var attributedString = new NSAttributedString ("Hello world", attrs);

			var customTest = new CustomNSAttributedString (attributedString);

			Console.WriteLine ("customTest.ClassHandle: {0}", customTest.ClassHandle);
		}
	}

	public class CustomNSAttributedString : NSMutableAttributedString
	{
		public CustomNSAttributedString (NSAttributedString other) : base (other)
		{
		}
	}
}