UserControl in Ribbon Unable to create Instance

When I reference my UserControl as content to a PopupButton in my Ribbon, the Designer says it can’t create an instance of it even though it works fine at run time.
My UserControl is a ListBox where I explicitly define all of the ListBoxItems in Xaml code. Each Item contains ribbon:Buttons and ribbon:ComboBox’s.
To get around the problem, I reference instead a property in the behind code, which creates the instance. The Designer is happy and I’m happy now!