Layout switching shortcut cannot be set through System Settings -> Keyboard

Yet another layout-switching-related bug...

[The bug]
The shortcut for layout switching cannont be changed through System Settings -> Keyboard -> Shortcuts -> Typing -> Switch to next source. Actually, some of the combinations can be set, like Ctrl+Space or Super+Space. And some combinations cannot. When you press, for example, Alt+Shift or Ctrl+Shift, the combination is not set and the status of the shortcut is displayed as "Disabled". And it is actually disabled, that is the layout switching shortcut doesn't work.
At the same time, it is quite possible to set these combinations through System Settings -> Text Entry -> Switch to next source using.

[How to reproduce]
1. Go to System Settings -> Keyboard -> Shortcuts.
2. Choose Typing item from the list.
3. Click on "Switch to next source" item in the list on the right.
4. You will see the invitation "New accelerator..." to type the shortcut.
5. Press Alt+Shift or Ctrl+Shift.

[What is expected]
The corresponding (Alt+Shift or Ctrl+Shift) combination should appear as a new shortcut.

[What happens]
"Disabled" status appears instead. The layout switching shortcut stops working.

[Current workaround]
Set the combination through System Settings -> Text Entry.

ProblemType: Bug
DistroRelease: Ubuntu 14.04
Package: unity-control-center 14.04.3+14.04.20140410-0ubuntu1
ProcVersionSignature: Ubuntu 3.13.0-24.46-generic 3.13.9
Uname: Linux 3.13.0-24-generic x86_64
ApportVersion: 2.14.1-0ubuntu3
Architecture: amd64
CurrentDesktop: Unity
Date: Sun Apr 27 16:55:01 2014
ExecutablePath: /usr/bin/unity-control-center
InstallationDate: Installed on 2012-05-02 (725 days ago)
InstallationMedia: Ubuntu 12.04 LTS "Precise Pangolin" - Release amd64 (20120425)
SourcePackage: unity-control-center
UpgradeStatus: Upgraded to trusty on 2014-04-21 (5 days ago)
usr_lib_unity-control-center:
 activity-log-manager 0.9.7-0ubuntu14
 deja-dup 30.0-0ubuntu4
 gnome-control-center 1:3.6.3-0ubuntu56