diff --git a/gtk/gtktoolbutton.c b/gtk/gtktoolbutton.c
index e7571b88c2..6232043f10 100644
--- a/gtk/gtktoolbutton.c
+++ b/gtk/gtktoolbutton.c
@@ -279,6 +279,10 @@ gtk_tool_button_init (GtkToolButton      *button,
 {
   GtkToolItem *toolitem = GTK_TOOL_ITEM (button);
 
+  /* We still need to use G_TYPE_INSTANCE_GET_PRIVATE() because GtkToolButton
+     need to access the class pointer inside instance_init
+     See a detailed explanation of this at
+     https://gitlab.gnome.org/GNOME/gtk/merge_requests/402#note_361210 */
   button->priv = G_TYPE_INSTANCE_GET_PRIVATE (button,
                                               GTK_TYPE_TOOL_BUTTON,
                                               GtkToolButtonPrivate);