Can't listen to system signals with gdbus

I’m trying to listen to a signal on the system bus using Gio’s dbus but it seems like I can only add callbacks to session bus signals. Callbacks on system signals are never called.
There’s no error or messages of any kind. It just… doesn’t run.

Am I missing something? Is there any extra requirements to listen to system signals?

        system_bus = Gio.bus_get_sync(Gio.BusType.SYSTEM, None)
        system_bus.signal_subscribe('org.freedesktop.login1',
                                    'org.freedesktop.login1.Manager',
                                    'PrepareForSleep',
                                    '/org/freedesktop/login1',
                                    None,
                                    Gio.DBusSignalFlags.NONE,
                                    on_prepare_for_sleep,
                                    None)
def on_prepare_for_sleep(connection, sender_name, object_path,
                         interface_name, signal_name, parameters,
                         *user_data):
    """Handle dbus prepare for sleep signal."""

    print('Called')

Thanks!