title: 'str' object has no attribute '_meta' error come when I uncomment admin.autodiscover()
body: <p>I am facing this error in django:</p>
  
  <pre><code>AttributeError at /
  'str' object has no attribute '_meta'
  Request Method: GET
  Request URL:    http://localhost:8000/
  Django Version: 1.3
  Exception Type: AttributeError
  Exception Value:    
  'str' object has no attribute '_meta'
  Exception Location: C:\Python27\lib\site-packages\django\contrib\admin\sites.py in         register, line 80
  Python Executable:  C:\Python27\python.exe
  Python Version: 2.7.2
  Python Path:    ['D:\\programming\\django_projects\\ecomstore',
  'C:\\Python27\\lib\\site-packages\\setuptools-0.6c9-py2.7.egg',
   'C:\\Python27\\lib\\site-packages\\pymysql-0.3-py2.6.egg',
   'C:\\Windows\\system32\\python27.zip',
   'C:\\Python27\\DLLs',
   'C:\\Python27\\lib',
   'C:\\Python27\\lib\\plat-win',
   'C:\\Python27\\lib\\lib-tk',
   'C:\\Python27',
   'C:\\Python27\\lib\\site-packages',
   'C:\\Python27\\lib\\site-packages\\PIL']
   Server time:   Fri, 13 Jan 2012 16:44:18 +0500
  </code></pre>
  
  <p>this error doesn't occur if I comment out <code>admin.autodiscover()</code>, also I used ModelForm in django so is it because of ModelForm? Following is ModelForm code:</p>
  
  <pre><code>from django import forms
  from catalog.models import Product
  
  class ProductAdminForm(forms.ModelForm):
    class Meta:
      model=Product
      def clean_price(self):
          if self.cleaned_data['price']&lt;=0:
              raise forms.ValidationError('Price must be greater than zero')
          return self.cleaned_data['price']
  </code></pre>
  
  <p>So if some one understand the problem then please tell.</p>
  
  <p>Following is traceback, pasted:</p>
  
  <pre><code>Environment:
  
  
   Request Method: GET
   Request URL: http://localhost:8000/
  
   Django Version: 1.3
   Python Version: 2.7.2
   Installed Applications:
   ['django.contrib.auth',
    'django.contrib.contenttypes',
    'django.contrib.sessions',
    'django.contrib.sites',
    'django.contrib.messages',
    'django.contrib.staticfiles',
    'django.contrib.admin',
    'ecomstore.catalog']
   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 "C:\Python27\lib\site-packages\django\core\handlers\base.py" in get_response
   101.                             request.path_info)
   File "C:\Python27\lib\site-packages\django\core\urlresolvers.py" in resolve
   250.             for pattern in self.url_patterns:
   File "C:\Python27\lib\site-packages\django\core\urlresolvers.py" in _get_url_patterns
   279.         patterns = getattr(self.urlconf_module, "urlpatterns", self.urlconf_module)
   File "C:\Python27\lib\site-packages\django\core\urlresolvers.py" in _get_urlconf_module
   274.             self._urlconf_module = import_module(self.urlconf_name)
   File "C:\Python27\lib\site-packages\django\utils\importlib.py" in import_module
   35.     __import__(name)
   File "D:\programming\django_projects\ecomstore\..\ecomstore\urls.py" in &lt;module&gt;
   5. admin.autodiscover()
   File "C:\Python27\lib\site-packages\django\contrib\admin\__init__.py" in autodiscover
   26.             import_module('%s.admin' % app)
   File "C:\Python27\lib\site-packages\django\utils\importlib.py" in import_module
   35.     __import__(name)
    File "D:\programming\django_projects\ecomstore\..\ecomstore\catalog\admin.py" in &lt;module&gt;
   19. admin.site.register('Product', 'ProductAdmin')
   File "C:\Python27\lib\site-packages\django\contrib\admin\sites.py" in register
   80.             if model._meta.abstract:
  
   Exception Type: AttributeError at /
   Exception Value: 'str' object has no attribute '_meta'
  </code></pre>