Comment 2 for bug 730211

I also have this error on subsequent Buildout run, after adding more dependencies to my project.

This time, this is due to zc.recipe.egg egg.py's Eggs.working_set() method:

           [...]
           if b_options.get('offline') == 'true':
>>> ws = zc.buildout.easy_install.working_set(
                   distributions, options['executable'],
                   [options['develop-eggs-directory'],
                    options['eggs-directory']],
                   include_site_packages=self.include_site_packages,
                   allowed_eggs_from_site_packages=self.allowed_eggs,
                   )
           else:
           [...]

And the Buildout function called:

       def working_set(specs, executable, path, include_site_packages=None,
                       allowed_eggs_from_site_packages=None, prefer_final=None):
           return install(
>>> specs, None, executable=executable, path=path,
               include_site_packages=include_site_packages,
               allowed_eggs_from_site_packages=allowed_eggs_from_site_packages,
               prefer_final=prefer_final)

And of course, the None here is the "dest" variable which triggers the error.