g_signal_list_ids() requires class initialization.  This means that for classes one has to call g_type_class_ref(type) and for interfaces g_type_default_interface_ref(type) beforehand to ensure the initialization.

However, for interfaces it can still be insufficient.  If the inspected interface registers no signals and no other class that would register signals have been initialized yet, it breaks.

An example will be attached.  In src/ it contains two interfaces:
- BadgerPesky (no signals)
- BadgerDomestic (has a signal)
and a program
- inspect

inspect essentially does

  g_type_default_interface_ref(type);
  ids = g_signal_list_ids(type, &nids);

for both interfaces.  If you run it

  ./src/inspect

it prints:

Inspecting BadgerPesky

(process:20020): GLib-GObject-WARNING **: gsignal.c:1063: unable to list signals of non instantiatable type `BadgerPesky'
0 signals:
Inspecting BadgerDomestic
1 signals: bite

By running it

   ./src/inspect invert

you instruct it to inspect the interfaces in the reverse order, and voilà, it works:

Inspecting BadgerDomestic
1 signals: bite
Inspecting BadgerPesky
0 signals:

There seems to be some global initialization missing in the first case.