django1.4.0：ImportError：没有名为base的模块

> OS：ubuntu 10.10
> Python：2.6.6
> Django：1.4 pre-alpha

之前,我为The Django Book安装了Django 1.1.1.今天,我通过cmd删除了Django 1.1.1：sudo apt-get install –purge python-django,然后获得最新版本的1.4 pre-alpha.

当我在docs.djangoproject.com上关注tutorial并执行cmd：python manage.py shell时,会出现一些错误,并且信息如下：

Traceback (most recent call last):
      File "manage.py",line 9,in <module>
        execute_from_command_line(sys.argv)
      File "/usr/local/lib/python2.6/dist-packages/django/core/management/__init__.py",line 422,in execute_from_command_line
        utility.execute()
      File "/usr/local/lib/python2.6/dist-packages/django/core/management/__init__.py",line 361,in execute
        self.fetch_command(subcommand).run_from_argv(self.argv)
      File "/usr/local/lib/python2.6/dist-packages/django/core/management/base.py",line 191,in run_from_argv
        self.execute(*args,**options.__dict__)
      File "/usr/local/lib/python2.6/dist-packages/django/core/management/base.py",line 222,in execute
        output = self.handle(*args,**options)
      File "/usr/local/lib/python2.6/dist-packages/django/core/management/base.py",line 355,in handle
        return self.handle_noargs(**options)
      File "/usr/local/lib/python2.6/dist-packages/django/core/management/commands/shell.py",line 46,in handle_noargs
        from django.db.models.loading import get_models
      File "/usr/local/lib/python2.6/dist-packages/django/db/__init__.py",line 27,in <module>
        connection = connections[DEFAULT_DB_ALIAS]
      File "/usr/local/lib/python2.6/dist-packages/django/db/utils.py",line 81,in __getitem__
        backend = load_backend(db['ENGINE'])
      File "/usr/local/lib/python2.6/dist-packages/django/db/utils.py",line 23,in load_backend
        return import_module('.base',backend_name)
      File "/usr/local/lib/python2.6/dist-packages/django/utils/importlib.py",line 35,in import_module
        __import__(name)
    ImportError: No module named base

有些人可以告诉我如何解决这个问题？

谢谢！！