At least for me, it appears to be because the 64-bit version is installed and the 32-bit version is not available:
unity-gtk-module-common:i386:
Installed: (none)
Candidate: (none)
Version table:
unity-gtk-module-common:
Installed: 0.0.0+13.10.20130716.1-0ubuntu1
Candidate: 0.0.0+13.10.20130716.1-0ubuntu1
Version table:
*** 0.0.0+13.10.20130716.1-0ubuntu1 0
500 [url="http://us.archive.ubuntu.com/ubuntu/"]http://us.archive.ubuntu.com/ubuntu/[/url] saucy/main amd64 Packages
100 /var/lib/dpkg/status
I ran the updater anyway. It started with the following error, but appears to be up