Django throws an exception when I add data from admin interface

It is a blog app, I have created a Article model and Tag model. when I add data to Article table from Admin interface, the data is stored to database, but if I click the link to change the entry, it throws an exception "string indices must be integers, not str", the traceback is as follows:
Environment:


Request Method: GET
Request URL: http://localhost:8000/admin/blog/article/1/

Django Version: 1.5.1
Python Version: 2.7.3
Installed Applications:
('django.contrib.auth',
 'django.contrib.contenttypes',
 'django.contrib.sessions',
 'django.contrib.sites',
 'django.contrib.messages',
 'django.contrib.staticfiles',
 'django.contrib.admin',
 'blog')
Installed Middleware:
('django.middleware.common.CommonMiddleware',
 'django.contrib.sessions.middleware.SessionMiddleware',
 'django.middleware.csrf.CsrfViewMiddleware',
 'django.contrib.auth.middleware.AuthenticationMiddleware',
 'django.contrib.messages.middleware.MessageMiddleware')


Traceback:
File "/home/yangyu/.virtualenvs/django-blog/local/lib/python2.7/site-packages/django/core/handlers/base.py" in get_response
  115.                         response = callback(request, *callback_args, **callback_kwargs)
File "/home/yangyu/.virtualenvs/django-blog/local/lib/python2.7/site-packages/django/contrib/admin/options.py" in wrapper
  372.                 return self.admin_site.admin_view(view)(*args, **kwargs)
File "/home/yangyu/.virtualenvs/django-blog/local/lib/python2.7/site-packages/django/utils/decorators.py" in _wrapped_view
  91.                     response = view_func(request, *args, **kwargs)
File "/home/yangyu/.virtualenvs/django-blog/local/lib/python2.7/site-packages/django/views/decorators/cache.py" in _wrapped_view_func
  89.         response = view_func(request, *args, **kwargs)
File "/home/yangyu/.virtualenvs/django-blog/local/lib/python2.7/site-packages/django/contrib/admin/sites.py" in inner
  202.             return view(request, *args, **kwargs)
File "/home/yangyu/.virtualenvs/django-blog/local/lib/python2.7/site-packages/django/utils/decorators.py" in _wrapper
  25.             return bound_func(*args, **kwargs)
File "/home/yangyu/.virtualenvs/django-blog/local/lib/python2.7/site-packages/django/utils/decorators.py" in _wrapped_view
  91.                     response = view_func(request, *args, **kwargs)
File "/home/yangyu/.virtualenvs/django-blog/local/lib/python2.7/site-packages/django/utils/decorators.py" in bound_func
  21.                 return func(self, *args2, **kwargs2)
File "/home/yangyu/.virtualenvs/django-blog/local/lib/python2.7/site-packages/django/db/transaction.py" in inner
  223.                 return func(*args, **kwargs)
File "/home/yangyu/.virtualenvs/django-blog/local/lib/python2.7/site-packages/django/contrib/admin/options.py" in change_view
  1112.             form = ModelForm(instance=obj)
File "/home/yangyu/.virtualenvs/django-blog/local/lib/python2.7/site-packages/django/forms/models.py" in __init__
  244.             object_data = model_to_dict(instance, opts.fields, opts.exclude)
File "/home/yangyu/.virtualenvs/django-blog/local/lib/python2.7/site-packages/django/forms/models.py" in model_to_dict
  129.                 data[f.name] = list(f.value_from_object(instance).values_list('pk', flat=True))
File "/home/yangyu/.virtualenvs/django-blog/local/lib/python2.7/site-packages/django/db/models/fields/related.py" in value_from_object
  1333.         return getattr(obj, self.attname).all()
File "/home/yangyu/.virtualenvs/django-blog/local/lib/python2.7/site-packages/django/db/models/manager.py" in all
  128.         return self.get_query_set()
File "/home/yangyu/.virtualenvs/django-blog/local/lib/python2.7/site-packages/django/db/models/fields/related.py" in get_query_set
  612.                 return self.instance._prefetched_objects_cache[self.prefetch_cache_name]

Exception Type: TypeError at /admin/blog/article/1/
Exception Value: string indices must be integers, not str


the code of Article model and Tag model is as follows:

class Tag(models.Model):
    name = models.CharField(max_length=50, unique=True, verbose_name='标签')
    slug = models.SlugField()

    def __unicode__(self):
        return self.name

    class Meta:
        verbose_name = '标签'
        verbose_name_plural = '标签'

    @models.permalink
    def get_absolute_url(self):
        return ('blog_tag', (), {'idx': self.id, 'slug': self.slug})


# Create your models here.
class Article(models.Model):
    """
    the model for blog article
    """
    STATUS_CHOICE = (
        (1, '编辑'),
        (2, '完成'),
        (3, '失效'),
    )
    title = models.CharField(max_length=100, verbose_name='标题')
    slug = models.SlugField(max_length=100)
    content = models.TextField(verbose_name='内容')
    created_time = models.DateTimeField(auto_now_add=True, verbose_name="创建时间")
    modified_time = models.DateTimeField(default=datetime.now,
                                         verbose_name="修改时间")
    tags = models.ManyToManyField(Tag, verbose_name='标签')
    category = models.ForeignKey('Category', blank=True, null=True, verbose_name='分类')
    always_top = models.BooleanField(default=False, verbose_name='置顶')
    author = models.CharField(max_length=40, verbose_name='作者')
    status = models.IntegerField(choices=STATUS_CHOICE,
                                 default=1, verbose_name='状态')

    @models.permalink
    def get_absolute_url(self):
        return ('blog_article', (), {'idx': self.id, 'slug': self.slug})

    def __getattr__(self, name):
        """
        add two attributes(summary, visible_comments) to the object
        Arguments:
        - `name`:
        """
        if name == 'summary':
            return util.get_summary(self.content)
        else:
            return ''

    def __unicode__(self):
        return self.title

    class Meta:
        ordering = ['-always_top', '-created_time']
        verbose_name = '博客文章'
        verbose_name_plural = '博客文章'

the problem seems to be caused by "tags = models.ManyToManyField(Tag, verbose_name='标签')" , when I delete this line, there is no problem, 
but when I add it to the model, django throws an expcetion, this problem really annoys me, who can help me?

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