Unable To Bind VisualBrush's Visual To ViewModel

Question

I have a TabControl whose ItemsSource property is bound to an ObservableCollection of ViewModels.

What I want is to have a Rectangle whose Fill is a VisualBrush - and binding the VisualBrush's Visual to the SelectedItem of the TabControl. When I do this, I get a binding error stating:

"'ObjectSourceConverter' converter failed to convert value WorkspaceViewModel' (type 'ViewModel'); fallback value will be used, if available. BindingExpression:Path=SelectedItem; DataItem='TabControl'"

Any idea on what I am doing incorrectly?

Thanks.

Chris