"Ctrl+c" text in menu item gets lost

[Phil Wolff <adiabat centurylink net>]

In 
gtkmm-documentation-master/examples/book/application/app_and_win_menus/exampleapplication.cc 
is this:

  Glib::RefPtr<Gio::MenuItem> item = Gio::MenuItem::create("_Copy", 
"win.copy");
  //Setting "accel" works, but might be deprecated soon: See 
https://bugzilla.gnome.org/show_bug.cgi?id=708908
  //When it is deprecated, use Gtk::Application::set_accel_for_action() 
instead.
  item->set_attribute_value("accel", 
Glib::Variant<Glib::ustring>::create("<Primary>c"));
  submenu_edit->append_item(item);

This code displays "Ctrl+c" in the menu item and the accelerator works 
as expected. However, when the suggested change is made:

  Glib::RefPtr<Gio::MenuItem> item = Gio::MenuItem::create("_Copy", 
"win.copy");
  set_accel_for_action("win.copy", "<Primary>c");
  submenu_edit->append_item(item);

the accelerator still works but the key combination hint is no longer 
shown in the menu.

Is there something else that needs to be done to restore the "Ctrl+c" 
part of the menu display?