[Zope] How to subclass plugin products in ZClass (zope 2.5.0x)

In Zope2.4.3 I can subclass a ZClass to the Photo product or the PhotoAlbum class. This I can't do in Zope2.5.0b3. When I try create a ZClass I can only subclass the buitlin products such as DTML Method, ZCatalog and so forth. Not Photo or PhotoFolder for example. Why? Is this a bug? Peter