New Post: Caliburn.Micro HelloScreens sample build error

Getting this error when I try to build the Caliburn.Micro sample project "Caliburn.Micro.HelloScreens" from http://caliburnmicro.codeplex.com Error: The type 'TDocument' must be a reference type in order to use it as parameter 'T' in the generic type or method 'Caliburn.Micro.Conductor'

TDocument is defined in the the class DocumentWorkspace as

public abstract class DocumentWorkspace<TDocument> : Conductor<TDocument>.Collection.OneActive, IDocumentWorkspace where TDocument : INotifyPropertyChanged, IDeactivate, IHaveDisplayName
{ }