Ubuntu – Cannot remap Alt-Tab combination after switching Ctrl and Alt

On Ubuntu 13.04 desktop I am trying to change keyboard layout.

First in System Settings -> Keyboard Layout -> Options -> Alt-Win behaviour,
I picked the following mapping:

Ctrl is mapped to Alt

Alt is mapped to Win.

And that works perfect.

Now I want to change shortcut for application switching (since I swapped keys it is Win–Tab now, I want it back on Alt–Tab)

In System Settings -> Keyboard -> Shortcuts -> Navigation,
I picked new combination for Switch Applications.

It is properly detected as Ctrl–Tab, but it is not working. It still works as Alt–Tab (since I swapped keyboard in first step, it is actually Win–Tab).

Is this a bug ? Is there any way I can change combination for switching applications ? Thanks.