It is very common libverto but no liberto-plugin is installed. We should at least display an error message in that case.
Proposed fix: http://fedorapeople.org/cgit/gd/public_git/gss-proxy.git/commit/?h=master-fixes2&id=d9db4e3
Fix pushed upstream.
Metadata Update from @gd: - Issue assigned to gd - Issue set to the milestone: 2013 May