gi.RepositoryError: Typelib file for namespace 'xlib', version '2.0' not found

Lubuntu (Ubuntu 12.04.1 LTS)
lubuntu-software-center: 0.0.5~bzr135-0ubuntu1

Trying to start Software-Center I get:

Traceback (most recent call last):
  File "/usr/bin/lubuntu-software-center", line 27, in <module>
    import LSC.main
  File "/usr/lib/python2.7/dist-packages/LSC/main.py", line 24, in <module>
    from gi.repository import Gtk
  File "/usr/lib/python2.7/dist-packages/gi/importer.py", line 76, in load_module
    dynamic_module._load()
  File "/usr/lib/python2.7/dist-packages/gi/module.py", line 222, in _load
    version)
  File "/usr/lib/python2.7/dist-packages/gi/module.py", line 90, in __init__
    repository.require(namespace, version)
gi.RepositoryError: Typelib file for namespace 'xlib', version '2.0' not found

This seems similar to bug report #857512. I create a new report because in this case the package is lubuntu-software-center.