R – Sivlerlight Spy cannot load ‘System.Windows.Controls.ChildWindow’

I'm currently trying out Silverlight Spy (http://silverlightspy.com) however it is showing a strange error when I use the ChildWindow control.

This error only shows up when I open my Silverlight app in Silverlight Spy.

I'm using Silverlight 3 Beta.

I've uploaded a project that reproduces the error for those who want to check: http://cid-095521bcc7670693.skydrive.live.com/self.aspx/Public/Code/SilverlightSpyTest.zip

Thanks!

Here is the error message:
[Warning] Script error occurred at line 1: Unhandled Error in Silverlight Application Could not load type 'System.Windows.Controls.ChildWindow' from assembly 'System.Windows.Controls, Version=2.0.5.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35'. at SilverlightSpyTest.MainPage.myButton_Click(Object sender, RoutedEventArgs e)