Can't set keyboard shortcuts to "none"

In Unity->Additional I can remap the keyboard shortcuts for the HUD and the dash, but I can't set then to "none" which is my preferred configuration.

ProblemType: Bug
DistroRelease: Ubuntu 13.04
Package: unity-tweak-tool 0.0.4
ProcVersionSignature: Ubuntu 3.8.0-19.29-generic 3.8.8
Uname: Linux 3.8.0-19-generic x86_64
NonfreeKernelModules: nvidia
ApportVersion: 2.9.2-0ubuntu8
Architecture: amd64
Date: Sat Apr 27 01:07:11 2013
InstallationDate: Installed on 2013-04-26 (0 days ago)
InstallationMedia: Ubuntu 13.04 "Raring Ringtail" - Release amd64 (20130424)
MarkForUpload: True
PackageArchitecture: all
SourcePackage: unity-tweak-tool
UpgradeStatus: No upgrade log present (probably fresh install)