diff --git a/gtk/gtkapplicationwindow.c b/gtk/gtkapplicationwindow.c
index 7174edc17c..41cbee949e 100644
--- a/gtk/gtkapplicationwindow.c
+++ b/gtk/gtkapplicationwindow.c
@@ -976,7 +976,7 @@ show_help_overlay (GSimpleAction *action,
 /**
  * gtk_application_window_set_help_overlay:
  * @window: a #GtkApplicationWindow
- * @help_overlay: a #GtkShortcutsWindow
+ * @help_overlay: (nullable): a #GtkShortcutsWindow
  *
  * Associates a shortcuts window with the application window, and
  * sets up an action with the name win.show-help-overlay to present