Plone 4.2 install error on Ubuntu 12.04 Precise & OS X Lio

This is the error I'm seeing when installing Plone 4.2 on Ubuntu 12.04 Precise and also on Mac OS X 10.7.4 Lion:

  Installing /home/ubuntu/.virtualenvs/plone/lib/python2.7/site-packages/grokcore.component-2.5-py2.7-nspkg.pth
  Running setup.py install for z3c.objpath

  Running setup.py install for zc.relation
    Traceback (most recent call last):
      File "/home/ubuntu/.virtualenvs/plone/lib/python2.7/site.py", line 682, in <module>
        main()
      File "/home/ubuntu/.virtualenvs/plone/lib/python2.7/site.py", line 662, in main
        paths_in_sys = addsitepackages(paths_in_sys)
      File "/home/ubuntu/.virtualenvs/plone/lib/python2.7/site.py", line 278, in addsitepackages
        addsitedir(sitedir, known_paths)
      File "/home/ubuntu/.virtualenvs/plone/lib/python2.7/site.py", line 203, in addsitedir
        addpackage(sitedir, name, known_paths)
      File "/home/ubuntu/.virtualenvs/plone/lib/python2.7/site.py", line 172, in addpackage
        exec(line)
      File "<string>", line 1, in <module>
    KeyError: 'z3c'
    Complete output from command /home/ubuntu/.virtualenvs/plone/bin/python -c "import setuptools;__file__='/home/ubuntu/.virtualenvs/plone/build/zc.relation/setup.py';exec(compile(open(__file__).read().replace('\r\n', '\n'), __file__, 'exec'))" install --single-version-externally-managed --record /tmp/pip-ezjGx_-record/install-record.txt --install-headers /home/ubuntu/.virtualenvs/plone/include/site/python2.7:
    Traceback (most recent call last):

  File "/home/ubuntu/.virtualenvs/plone/lib/python2.7/site.py", line 682, in <module>

    main()

  File "/home/ubuntu/.virtualenvs/plone/lib/python2.7/site.py", line 662, in main

    paths_in_sys = addsitepackages(paths_in_sys)

  File "/home/ubuntu/.virtualenvs/plone/lib/python2.7/site.py", line 278, in addsitepackages

    addsitedir(sitedir, known_paths)

  File "/home/ubuntu/.virtualenvs/plone/lib/python2.7/site.py", line 203, in addsitedir

    addpackage(sitedir, name, known_paths)

  File "/home/ubuntu/.virtualenvs/plone/lib/python2.7/site.py", line 172, in addpackage

    exec(line)

  File "<string>", line 1, in <module>

KeyError: 'z3c'

----------------------------------------
Command /home/ubuntu/.virtualenvs/plone/bin/python -c "import setuptools;__file__='/home/ubuntu/.virtualenvs/plone/build/zc.relation/setup.py';exec(compile(open(__file__).read().replace('\r\n', '\n'), __file__, 'exec'))" install --single-version-externally-managed --record /tmp/pip-ezjGx_-record/install-record.txt --install-headers /home/ubuntu/.virtualenvs/plone/include/site/python2.7 failed with error code 1 in /home/ubuntu/.virtualenvs/plone/build/zc.relation
Storing complete log in /home/ubuntu/.pip/pip.log