New Post: AvalonDock ViewModels remain in memory after being removed form DocumentsSource

Hi,
I am using AvalonDock in an MVVM application. I have DocumetsSource property bound to an ObservableCollection<WorkspaceViewModel>. WorkspaceViewModels can be successfully added and removed from the collection and the tab item in AvalonDock is added removed as expected. However, the WorkspaceViewModels always remain in memory.

I have created a sample project here

Is there something I'm doing wrong?