ApplicationMenu binding ItemsSource and using ItemTemplateSelector

I have an ApplicationMenu that I'm binding the ItemsSource to an ObservableCollection and I know that the binding is happening correctly because I see an item in the menu for each item in the list however it seems that setting the ItemTemplateSelector is having no effect.

<ribbon:ApplicationMenu ItemsSource="{Binding MenuItems}"
                        ItemTemplateSelector="{StaticResource AppMenuTemplateSelector}"/>

 The breakpoint in the template selector is never hit.

Ben