Pelican slug sorting issue

pelican-slug-issue

DEBUG: Adding current directory to system path
DEBUG: Temporarily adding PLUGIN_PATHS to system path
DEBUG: Restoring system path
DEBUG: Template list: ['!simple/archives.html', '!simple/article.html', '!simple/author.html', '!simple/authors.html', '!simple/base.html', '!simple/categories.html', '!simple/category.html', '!simple/gosquared.html', '!simple/index.html', '!simple/page.html', '!simple/pagination.html', '!simple/period_archives.html', '!simple/tag.html', '!simple/tags.html', '!simple/translations.html', '404.html', 'archives.html', 'article.html', 'author.html', 'authors.html', 'base.html', 'categories.html', 'category.html', 'gosquared.html', 'includes/404-content.html', 'includes/footer.html', 'includes/id-common.html', 'includes/id-twitter.html', 'includes/item-header.html', 'index.html', 'page.html', 'pagination.html', 'period_archives.html', 'tag.html', 'tags.html', 'translations.html']
DEBUG: Read file 0-01.md -> Article
DEBUG: Signal article_generator_preread.send(ArticlesGenerator)
DEBUG: Successfuly imported extension module "markdown.extensions.extra".
<snip: all the extensions import successfully>
DEBUG: Read file 0-02.md -> Article
<snip other 0.x files>
DEBUG: Read file 0-14.md -> Article
DEBUG: Read file 1-01.md -> Article
<snip other 1.x files>
DEBUG: Read file 1-18.md -> Article
DEBUG: Read file 2-01.md -> Article
<snip other 2.x files>
DEBUG: Read file 2-15.md -> Article
CRITICAL: unorderable types: list() < str()
Traceback (most recent call last):
  File "/Users/chris/.pyenv/versions/pelican/bin/pelican", line 11, in <module>
    sys.exit(main())
  File "/Users/chris/.pyenv/versions/pelican/lib/python3.4/site-packages/pelican/__init__.py", line 425, in main
    pelican.run()
  File "/Users/chris/.pyenv/versions/pelican/lib/python3.4/site-packages/pelican/__init__.py", line 169, in run
    p.generate_context()
  File "/Users/chris/.pyenv/versions/pelican/lib/python3.4/site-packages/pelican/generators.py", line 533, in generate_context
    order_by=self.settings['ARTICLE_ORDER_BY'])
  File "/Users/chris/.pyenv/versions/pelican/lib/python3.4/site-packages/pelican/utils.py", line 462, in process_translations
    content_list.sort(key=attrgetter('slug'))
TypeError: unorderable types: list() < str()