Werkzeug Internal Server Error

traceback.txt

Traceback (most recent call last):
  File "/mnt/share/code/git/review/venv/lib/python2.7/site-packages/flask/app.py", line 1836, in __call__
    return self.wsgi_app(environ, start_response)
  File "/mnt/share/code/git/review/venv/lib/python2.7/site-packages/flask/app.py", line 1820, in wsgi_app
    response = self.make_response(self.handle_exception(e))
  File "/mnt/share/code/git/review/venv/lib/python2.7/site-packages/flask/app.py", line 1403, in handle_exception
    reraise(exc_type, exc_value, tb)
  File "/mnt/share/code/git/review/venv/lib/python2.7/site-packages/flask/app.py", line 1817, in wsgi_app
    response = self.full_dispatch_request()
  File "/mnt/share/code/git/review/venv/lib/python2.7/site-packages/flask/app.py", line 1477, in full_dispatch_request
    rv = self.handle_user_exception(e)
  File "/mnt/share/code/git/review/venv/lib/python2.7/site-packages/flask/app.py", line 1381, in handle_user_exception
    reraise(exc_type, exc_value, tb)
  File "/mnt/share/code/git/review/venv/lib/python2.7/site-packages/flask/app.py", line 1475, in full_dispatch_request
    rv = self.dispatch_request()
  File "/mnt/share/code/git/review/venv/lib/python2.7/site-packages/flask/app.py", line 1461, in dispatch_request
    return self.view_functions[rule.endpoint](**req.view_args)
  File "/mnt/share/code/git/review/review/views.py", line 38, in review_list
    return render_template('list.html', reviews=reviews)
  File "/mnt/share/code/git/review/venv/lib/python2.7/site-packages/flask/templating.py", line 128, in render_template
    context, ctx.app)
  File "/mnt/share/code/git/review/venv/lib/python2.7/site-packages/flask/templating.py", line 110, in _render
    rv = template.render(context)
  File "/mnt/share/code/git/review/venv/lib/python2.7/site-packages/jinja2/environment.py", line 969, in render
    return self.environment.handle_exception(exc_info, True)
  File "/mnt/share/code/git/review/venv/lib/python2.7/site-packages/jinja2/environment.py", line 742, in handle_exception
    reraise(exc_type, exc_value, tb)
  File "/mnt/share/code/git/review/review/templates/list.html", line 1, in top-level template code
    {% extends "base.html" %}
  File "/mnt/share/code/git/review/review/templates/base.html", line 63, in top-level template code
    {% block body %}
  File "/mnt/share/code/git/review/review/templates/base.html", line 86, in block "body"
    {% block content %}
  File "/mnt/share/code/git/review/review/templates/list.html", line 15, in block "content"
    <p class="pull-right">{{ review.overall_review.stars|show_stars|safe }}</p>
  File "/mnt/share/code/git/review/review/filters.py", line 2, in show_stars
    return ''.join(['<img data-src="/static/svg/smart/star.svg" class="iconic iconic-sm">' for _ in range(score)])
UndefinedError: 'None' has no attribute 'stars'