alt+shift can't be set to toggle language [duplicate]

I recently did a fresh installation of Ubuntu 13.10; but there is something bothering me, which I don't quite understand.

When I first tried to toggle the keyboard language(I usually switch between Persian and English) using the good old "Alt+Shift" shortcut it didn't work. Then, I went and checked the Keyboard shortcut settings and found out that it had been set to "Super+Space"(which BTW didn't work either). So I tried to change it back to "Alt+Shift" but it just doesn't work; when I press "Alt+Shift" to set it up as the toggle-language shortcut, the box automatically resets itself to its previous value(without any errors whatsoever). As far as I've checked I couldn't find any thing obvious corresponding to the shortcut "Alt+Shift" either.

I've currently set up the shortcut as "Ctrl+Space"; so I can toggle the language. My question is why I cannot set it up to just "Alt+Shift"?

I had the same problem after upgrading to Saucy.

Here's how I fixed it. First you need dconf-editor, so just install it by typing (in a terminal):

sudo apt-get install dconf-editor

After installed run it by typing

dconf-editor

In the new window browse to Desktop -> ibus -> General -> Hotkey
then change the next-engine & next-engine-in-menu values to something like this:

['Shift_L+Alt']  

Close the window and go to the text entry settings.
They should now allow you to use Alt+Shift

if the above doesn't help, try this :

sudo add-apt-repository ppa:attente/1218322
sudo apt-get update
sudo apt-get upgrade

then restart.

To set Shift+Alt combination as your language switcher use the following:

Tweak Tool -> Typing -> Key(s) to change layout

This will change the layout using whatsoever key combination you chose from the menu. But it will not change the Layout indicator in Ubuntu 13.10 (Unity)

Install Tweak Tool from Ubuntu Software Center