title: DoesNotExist at /accounts/register/ Site matching query does not exist. (django, python)
body: <p>Trying again to implement django-registration. When I try to deploy it to the heroku and to register a new user, it gives me a strange error:</p>
  
  <pre><code>Traceback:
  
  #some irrelevant traceback
  
  File "/app/.heroku/python/lib/python2.7/site-packages/registration/views.py" in post
    43.             return self.form_valid(request, form)
  
  File "/app/.heroku/python/lib/python2.7/site-packages/registration/views.py" in form_valid
    91.         new_user = self.register(request, form)
  
  File "/app/.heroku/python/lib/python2.7/site-packages/registration/backends/default/views.py" in register
    86.         site = get_current_site(request)
  
  File "/app/.heroku/python/lib/python2.7/site-packages/django/contrib/sites/shortcuts.py" in get_current_site
    15.         return Site.objects.get_current(request)
  
  File "/app/.heroku/python/lib/python2.7/site-packages/django/contrib/sites/models.py" in get_current
    67.             return self._get_site_by_request(request)
  
  File "/app/.heroku/python/lib/python2.7/site-packages/django/contrib/sites/models.py" in _get_site_by_request
    44.                 SITE_CACHE[host] = self.get(domain__iexact=host)
  
  File "/app/.heroku/python/lib/python2.7/site-packages/django/db/models/manager.py" in manager_method
    122.                 return getattr(self.get_queryset(), name)(*args, **kwargs)
  
  File "/app/.heroku/python/lib/python2.7/site-packages/django/db/models/query.py" in get
    387.                 self.model._meta.object_name
  </code></pre>
  
  <hr>
  
  <pre><code>Exception Type: DoesNotExist at /accounts/register/
  Exception Value: Site matching query does not exist.
  </code></pre>
  
  <p>I am following exactly the official <a href="http://django-registration.readthedocs.io/en/2.1.1/install.html" rel="noreferrer">documentation</a>. </p>
  
  <p>Where should look for the error? Where is the problem?</p>