diff --git a/gtk/gtkenums.h b/gtk/gtkenums.h
index b0c7b9430d..83fb4c645e 100644
--- a/gtk/gtkenums.h
+++ b/gtk/gtkenums.h
@@ -1368,6 +1368,12 @@ typedef enum {
   GTK_ACCESSIBLE_STATE_SELECTED
 } GtkAccessibleState;
 
+/**
+ * GTK_ACCESSIBLE_VALUE_UNDEFINED:
+ *
+ * An undefined value. The accessible attribute is either unset, or its
+ * value is undefined.
+ */
 #define GTK_ACCESSIBLE_VALUE_UNDEFINED  (-1)
 
 /**