DictionaryKeyProperty attribute does not appear to be usable.

I'm trying to add a special class to be used in a ResourceDictionary. It's using a custom key, and as per the WPF documentation I  am using the DictionaryKeyPropertyAttribute on my new class type to specify a custom key property.

Here is the code:

using System;

using System.Collections.Generic;

using System.Linq;

using System.Text;

using System.Windows.Markup;

namespace DictionaryKeyPropertyBug

{

[DictionaryKeyProperty("Property1")]

public class DictionaryKeyPropertyTestClass

{

public object Property1 { get; set; }

}

}

And the XAML:

<Window x:Class="DictionaryKeyPropertyBug.Window1"

xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"

xmlns:local="clr-namespace:DictionaryKeyPropertyBug"

xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">

<Window.Resources>

<local:DictionaryKeyPropertyTestClass Property1="foo"/>

</Window.Resources>

</Window>

And here is the error I get:
Error 1 All objects added to an IDictionary must have a Key attribute or some other type of key associated with them. Line 6 Position 10. Window1.xaml 6 10 DictionaryKeyPropertyBug

It's telling me I need a key (e.g. x:Key="..."), but I shouldn't because the purpose of the DictionaryKeyPropertyAttribute to define what property the key represents. In this case Property1 is the key.

Anyone know what is going on here?