AccelMap.add_entry not working for CTRL+TAB

Doing the following seems to have no effect. The default CTRL+TAB
mapping stays is executed (i.e. changes to another child window).

Gtk::AccelMap.add_entry(’/switch’, Gdk::Keyval::GDK_Tab,

I’ve tested with other key combinations and they work fine. I could use
key_press_event but that means I’d have to code a special case just for
this problem.

Any ideas?