WPF ListBox + Binding + IDataErrorInfo = ?

Question

I have a WPF MVVM application. In the View I have a multiselect ListBox. In the ViewModel I have a property for the selected items in the List.

Using a technique similar to the one in the answer to this question I can bind my property to the ListBox.

But I also want to do data validation via IDataErrorInfo. All I want to do is check that the user has selected at least one item in the list. Adding ValidatesOnDataErrors=True to the binding doesn't work.

Is there any way to have a multiselect listbox that's databound with IDataErrorInfo?