[glade--] get_widget_derived() on indirect base class

I've got a class I've created which is derived from Gtk::EventBox and 
then another class derived from that:

class Chip: public Gtk::EventBox;
class EditChip: public Chip;

get_widget_derived() complains that Gtk::EventBox is not a direct or 
virtual base of EditChip.  Is there a way to get around this or do I 
have to inherit from eventbox a second time in my second class?