There's an... interesting bit of logic in utils/misc.py:

We'd likely want to change it to:

if not virtualenv_no_global(): 
    sites.append(site_packages)
    ...

None the less, if tests fail with that, change it to:

# NOTE: what about virtualenvs with no global site-packages
sites.append(site_packages)
...

Good First Issue: This issue is a good starting point for first time contributors -- the process of fixing this should be a good introduction to pip's development workflow. If you've already contributed to pip, work on another issue without this label instead. If there is not a corresponding pull request for this issue, it is up for grabs. For directions for getting set up, see our Getting Started Guide. If you are working on this issue and have questions, feel free to ask them here, #pypa-dev on Freenode, or the pypa-dev mailing list.