title: g_dbus_proxy_new_for_bus_sync: assertion 'g_variant_is_object_path (object_path)' failed
body: <p>The code following code should retrieve systemd-logind <code>IdleSinceHint</code> property exposed on DBus</p>
  
  <pre><code>/*
   * Compile with:
   *   gcc -Wall print_user_idle_time.c -o print_user_idle_time `pkg-config --libs gio-2.0 --cflags`
   */
  
  #include &lt;gio/gio.h&gt;
  
  static void
  print_user_idle_time (GDBusProxy *proxy)
  {
      gchar *property = "IdleSinceHint";
      GError *error = NULL;
      GVariant *ret;
      guint64 user_idle_time;
  
      ret = g_dbus_proxy_get_cached_property(proxy, property);
      if (!ret) {
          g_dbus_error_strip_remote_error (error);
          g_print ("IdleSinceHint failed: %s\n", error-&gt;message);
          g_error_free (error);
          return;
      }
  
      g_variant_get (ret, "(^ao)", &amp;user_idle_time);
      g_print("%lu\n", user_idle_time);
      g_variant_unref (ret);
  }
  
  int
  main (int argc, char *argv[])
  {
      GDBusProxy *proxy;
      gchar *name = "org.freedesktop.login1";
      gchar *object_path = "org/freedesktop/login1";
      gchar *interface_name = "org.freedesktop.login1.Manager";
      /* Create a D-Bus proxy */
      proxy = g_dbus_proxy_new_for_bus_sync (G_BUS_TYPE_SYSTEM,
                                             G_DBUS_PROXY_FLAGS_NONE,
                                             NULL,
                                             name,
                                             object_path,
                                             interface_name,
                                             NULL, NULL);
      g_assert (proxy != NULL);
  
      print_user_idle_time (proxy);
  
      g_object_unref (proxy);
  
      return 0;
  }
  </code></pre>
  
  <p>but when it is runned it fails on assertion <code>g_assert (proxy != NULL);</code> with error</p>
  
  <pre><code>(process:9059): GLib-GIO-CRITICAL **: 17:29:07.245: g_dbus_proxy_new_for_bus_sync: assertion 'g_variant_is_object_path (object_path)' failed
  **
  ERROR:print_user_idle_time.c:44:main: assertion failed: (proxy != NULL)
  </code></pre>
  
  <p>What can be the problem? Thank you</p>