TemplateInstance.Single breaks aspx designer file

Question

In Visual Studio 2008 I am trying to modify a UserControl to support TemplatInstance.Single. However, with that value the IDE no longer is able to update the designer file when new controls are added -- any controls, not just those inside the template of the UserControl.

[PersistenceMode(PersistenceMode.InnerProperty)]
[Browsable(false)]
[TemplateContainer(typeof (ExpandCollapsePanelContainer))]
[TemplateInstance(TemplateInstance.Multiple)]
public ITemplate Contents { get; set; }

...

public class ExpandCollapsePanelContainer : Control, INamingContainer
{
}

If I use 'Multiple' or remove the attribute entirely, the control behaves properly. It's only the 'Single' value that is causing the problem.

Anyone have any ideas?

Thanks.

Colin