diff --git a/gtk/gtkactionobserver.c b/gtk/gtkactionobserver.c
index 77faa7fcb7..96791a9a27 100644
--- a/gtk/gtkactionobserver.c
+++ b/gtk/gtkactionobserver.c
@@ -158,15 +158,17 @@ gtk_action_observer_action_removed (GtkActionObserver   *observer,
  * gtk_action_observer_primary_accel_changed:
  * @observer: a `GtkActionObserver`
  * @observable: the source of the event
- * @action_name: the name of the action
- * @action_and_target: detailed action of the changed accel, in “action and target” format
+ * @action_name: (nullable): the name of the action
+ * @action_and_target: (nullable): detailed action of the changed accel, in “action and target” format
  *
  * This function is called when an action that the observer is
  * registered to receive events for has one of its accelerators changed.
  *
  * Accelerator changes are reported for all targets associated with the
- * action.  The @action_and_target string should be used to check if the
+ * action. The @action_and_target string should be used to check if the
  * reported target is the one that the observer is interested in.
+ *
+ * Either @action_name or @action_and_target may be %NULL.
  */
 void
 gtk_action_observer_primary_accel_changed (GtkActionObserver   *observer,