[Vala] DBus interface declaration not possible in module/plugin

[JM <interflug1 gmx net>]

Hello
In my application there is one plugin/module using a interface for
connecting to a dbus service. 
This is working perfectly fine as long as the dbus interface is declared
on the application side. But as this interface should not be in the
public API of the application, I'd like to shift the interface to the
module itself.

If I do it like that the program compiles fine, but I'm getting a crash
at runtime with the message:
GLib-GObject:ERROR:/build/buildd/glib2.0-2.32.3/./gobject/gtype.c:1945:type_iface_ensure_dflt_vtable_Wm: 
assertion failed: (iface->data)


So this seems to be valid for dbus clients(!) in loadable modules: The
dbus
interface has to be declared on the application side and (therefore) be
public.

Is there a way around that?
Can I register that (dbus)interface in a dynamic way on the
plugin/module side?

Thanks in advance!

Best Regards
JM