So, this test is currently not passing, and I don't understand why. It checks that a `ValueError` thrown by the serializer results in a 500 error, and that the 500 error contains the correct CORS stuff. Ok. But when I run it:
```
======================================================================
ERROR: test_other_cors (mkt.webpay.tests.test_resources.TestPrices)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/mat/.virtualenvs/zamboni26/lib/python2.6/site-packages/mock.py", line 1201, in patched
    return func(*args, **keywargs)
  File "/home/mat/work/mozilla/zamboni/mkt/webpay/tests/test_resources.py", line 181, in test_other_cors
    res = self.client.get(self.get_url)
  File "/home/mat/work/mozilla/zamboni/mkt/api/tests/test_oauth.py", line 90, in get
    .get(url, **self.kw(headers, **kw)))
  File "/home/mat/.virtualenvs/zamboni26/lib/python2.6/site-packages/django/test/client.py", line 439, in get
    response = super(Client, self).get(path, data=data, **extra)
  File "/home/mat/.virtualenvs/zamboni26/lib/python2.6/site-packages/django/test/client.py", line 244, in get
    return self.request(**r)
  File "/home/mat/.virtualenvs/zamboni26/lib/python2.6/site-packages/django/test/client.py", line 381, in request
    response = self.handler(environ)
  File "/home/mat/.virtualenvs/zamboni26/lib/python2.6/site-packages/django/test/client.py", line 84, in __call__
    response = self.get_response(request)
  File "/home/mat/.virtualenvs/zamboni26/lib/python2.6/site-packages/django/core/handlers/base.py", line 179, in get_response
    response = self.handle_uncaught_exception(request, resolver, sys.exc_info())
  File "/home/mat/.virtualenvs/zamboni26/lib/python2.6/site-packages/django/core/handlers/base.py", line 111, in get_response
    response = callback(request, *callback_args, **callback_kwargs)
  File "/home/mat/.virtualenvs/zamboni26/lib/python2.6/site-packages/rest_framework/viewsets.py", line 78, in view
    return self.dispatch(request, *args, **kwargs)
  File "/home/mat/.virtualenvs/zamboni26/lib/python2.6/site-packages/django/views/decorators/csrf.py", line 77, in wrapped_view
    return view_func(*args, **kwargs)
  File "/home/mat/.virtualenvs/zamboni26/lib/python2.6/site-packages/rest_framework/views.py", line 399, in dispatch
    response = self.handle_exception(exc)
  File "/home/mat/.virtualenvs/zamboni26/lib/python2.6/site-packages/rest_framework/views.py", line 396, in dispatch
    response = handler(request, *args, **kwargs)
  File "/home/mat/.virtualenvs/zamboni26/lib/python2.6/site-packages/rest_framework/mixins.py", line 106, in retrieve
    return Response(serializer.data)
  File "/home/mat/.virtualenvs/zamboni26/lib/python2.6/site-packages/rest_framework/serializers.py", line 537, in data
    self._data = self.to_native(obj)
  File "/home/mat/.virtualenvs/zamboni26/lib/python2.6/site-packages/rest_framework/serializers.py", line 325, in to_native
    value = field.field_to_native(obj, field_name)
  File "/home/mat/.virtualenvs/zamboni26/lib/python2.6/site-packages/rest_framework/fields.py", line 1008, in field_to_native
    value = getattr(self.parent, self.method_name)(obj)
  File "/home/mat/.virtualenvs/zamboni26/lib/python2.6/site-packages/mock.py", line 955, in __call__
    return _mock_self._mock_call(*args, **kwargs)
  File "/home/mat/.virtualenvs/zamboni26/lib/python2.6/site-packages/mock.py", line 1010, in _mock_call
    raise effect
ValueError:
```

It used to pass with the tastypie version. What am I doing wrong?