giomm DBus service throwing errors on connection establishment

[Jonas Platte <jonasplatte myopera com>]

Hello,
I don't know if this is the right place to ask but I found nothing 
especially about giomm.
I'm trying to write a basic dbus service. The methods work, but as soon 
as I use properties I get the following error once per property upon 
connection establishment (through a Gio::DBus::Proxy in a sample program):

(process:11770): GLib-CRITICAL **: g_variant_ref_sink: assertion `value 
!= NULL' failed

When I try to access the properties through d-feet, the service crashes 
with following output:

(process:12556): GLib-CRITICAL **: g_variant_ref_sink: assertion `value 
!= NULL' failed
**
GLib-GIO:ERROR:/build/buildd/glib2.0-2.36.0/./gio/gdbusconnection.c:4096:invoke_get_property_in_idle_cb: 
assertion failed: (error != NULL)

The property_get slot is set and the function also gets called, but I 
don't know how to return a value in this function (the only glibmm dbus 
example I found is http://pastebin.com/YWrC615R and the documentation 
didn't help me either).

How are SlotInterfaceGetProperty and SlotInterfaceSetProperty supposed 
to be used?