我正在运行virtualenv尝试学习Django，但是由于安装Django之后的任何原因以及当我尝试访问默认的Django起始页面时，在浏览器中出现以下错误：

发生服务器错误。请与管理员联系。

在我运行服务器的终端窗口中，显示以下错误：

配置不当：模块“ django.contrib.auth.middleware”未定义“
SessionAuthenticationMiddleware”属性/类

如果有人对我为什么在virtualenv中遇到此错误有任何见解，我将不胜感激。不过，我可以使服务器在非virtualenv设置中正常运行。

这是完整的堆栈跟踪：

Traceback (most recent call last):
  File "/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/wsgiref/handlers.py", line 85, in run
    self.result = application(self.environ, self.start_response)
  File "/Users/jruecke/Python/JSON/lib/python2.7/site-packages/Django-1.6.5-py2.7.egg/django/core/handlers/wsgi.py", line 187, in __call__
    self.load_middleware()
  File "/Users/jruecke/Python/JSON/lib/python2.7/site-packages/Django-1.6.5-py2.7.egg/django/core/handlers/base.py", line 45, in load_middleware
    mw_class = import_by_path(middleware_path)
  File "/Users/jruecke/Python/JSON/lib/python2.7/site-packages/Django-1.6.5-py2.7.egg/django/utils/module_loading.py", line 31, in import_by_path
    error_prefix, module_path, class_name))
ImproperlyConfigured: Module "django.contrib.auth.middleware" does not define a "SessionAuthenticationMiddleware" attribute/class
[16/Sep/2014 22:44:30] "GET / HTTP/1.1" 500 59
Traceback (most recent call last):
  File "/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/wsgiref/handlers.py", line 85, in run
    self.result = application(self.environ, self.start_response)
  File "/Users/jruecke/Python/JSON/lib/python2.7/site-packages/Django-1.6.5-py2.7.egg/django/core/handlers/wsgi.py", line 187, in __call__
    self.load_middleware()
  File "/Users/jruecke/Python/JSON/lib/python2.7/site-packages/Django-1.6.5-py2.7.egg/django/core/handlers/base.py", line 45, in load_middleware
    mw_class = import_by_path(middleware_path)
  File "/Users/jruecke/Python/JSON/lib/python2.7/site-packages/Django-1.6.5-py2.7.egg/django/utils/module_loading.py", line 31, in import_by_path
    error_prefix, module_path, class_name))
ImproperlyConfigured: Module "django.contrib.auth.middleware" does not define a "SessionAuthenticationMiddleware" attribute/class