python - セッションでデータを保存しようとしている本番サーバーで TypeError が発生するのはなぜですか?

Question

環境：

Request Method: GET
Request URL: django_server_running_on_apache

Django Version: 1.3.3
Python Version: 2.7.3

Traceback:
File "/usr/local/lib/python2.7/dist-packages/django/core/handlers/base.py" in get_response
  178.                 response = middleware_method(request, response)
File "/usr/local/lib/python2.7/dist-packages/django/contrib/sessions/middleware.py" in process_response
  36.                 request.session.save()
File "/usr/local/lib/python2.7/dist-packages/django/contrib/sessions/backends/file.py" in save
  121.                     os.write(output_file_fd, self.encode(session_data))
File "/usr/local/lib/python2.7/dist-packages/django/contrib/sessions/backends/base.py" in encode
  93.         pickled = pickle.dumps(session_dict, pickle.HIGHEST_PROTOCOL)

Exception Type: TypeError at /
Exception Value: expected string or Unicode object, NoneType found