Why oh why can't I get the following working :-) (Studio2005)

I'm trying to use ToolboxItemFilter to restrict what controls are
available when desiging a custom control. So I've a trivial example
application:

 [ToolboxItemFilter("Test",ToolboxItemFilterType.Require)]
 public class MyDesigner : DocumentDesigner { }

 [Designer(typeof(MyDesigner),typeof(IRootDesigner))]
 [ToolboxItemFilter("Test")]
 public class MyBase : Panel { }

 [ToolboxItemFilter( "Test"  )]
 public class SomeControl : Panel { }

Then add a 'UserControl' to the project; change its base class to be
MyBase, re-compile, and edit in the IDE.

There are NO items available in the toolbox.

Comment out the line
 [ToolboxItemFilter("Test",ToolboxItemFilterType.Require)]
and all the control appear again.

So, it's failing to notice the Filter attribute on the MyBase and
SomeControl items....

Any ideas? I can't see what I'm doing differently to the sample
application (shape designer) on MSDN, really..