commented lines that leads to build failure:

$ mkdocs build
Building documentation to directory: site
Traceback (most recent call last):
  File "/usr/bin/mkdocs", line 46, in <module>
    main(cmd, args=sys.argv[2:], options=dict(opts))
  File "/usr/bin/mkdocs", line 32, in main
    build(config)
  File "/usr/lib/python2.7/site-packages/mkdocs/build.py", line 179, in build
    build_pages(config)
  File "/usr/lib/python2.7/site-packages/mkdocs/build.py", line 164, in build_pages
    output_content = template.render(context)
  File "/usr/lib/python2.7/site-packages/jinja2/environment.py", line 969, in render
    return self.environment.handle_exception(exc_info, True)
  File "/usr/lib/python2.7/site-packages/jinja2/environment.py", line 742, in handle_exception
    reraise(exc_type, exc_value, tb)
  File "./theme/mkdocs/base.html", line 47, in top-level template code
    {% include "nav.html" %}
  File "./theme/mkdocs/nav.html", line 7, in top-level template code
    {% for first_item in menu.children[:1] %}