カスタムモデルフィールド（および付随するフォームフィールド）を含むDjangoアプリケーションがあります。組み込みの開発サーバーを使用してプロジェクトを実行すると、正常に動作します。ただし、サーバーにgunicornをロードすると、次のエラーが発生します。
TemplateSyntaxError at /admin/
Caught TypeError while rendering: Error when calling the metaclass bases
must be type, not None
完全を期すために、以下にエラー全体を含めました。これは、障害のある行を示しています。
Traceback:
File "/home/rob/.virtualenvs/shipp/lib/python2.7/site-packages/django/core/handlers/base.py" in get_response
111. response = callback(request, *callback_args, **callback_kwargs)
File "/home/rob/.virtualenvs/shipp/lib/python2.7/site-packages/django/contrib/admin/sites.py" in wrapper
214. return self.admin_view(view, cacheable)(*args, **kwargs)
File "/home/rob/.virtualenvs/shipp/lib/python2.7/site-packages/django/utils/decorators.py" in _wrapped_view
93. response = view_func(request, *args, **kwargs)
File "/home/rob/.virtualenvs/shipp/lib/python2.7/site-packages/django/views/decorators/cache.py" in _wrapped_view_func
79. response = view_func(request, *args, **kwargs)
File "/home/rob/.virtualenvs/shipp/lib/python2.7/site-packages/django/contrib/admin/sites.py" in inner
197. return view(request, *args, **kwargs)
File "/home/rob/.virtualenvs/shipp/lib/python2.7/site-packages/django/views/decorators/cache.py" in _wrapped_view_func
79. response = view_func(request, *args, **kwargs)
File "/home/rob/.virtualenvs/shipp/lib/python2.7/site-packages/django/contrib/admin/sites.py" in index
382. context_instance=context_instance
File "/home/rob/.virtualenvs/shipp/lib/python2.7/site-packages/django/shortcuts/__init__.py" in render_to_response
20. return HttpResponse(loader.render_to_string(*args, **kwargs), **httpresponse_kwargs)
File "/home/rob/.virtualenvs/shipp/lib/python2.7/site-packages/django/template/loader.py" in render_to_string
188. return t.render(context_instance)
File "/home/rob/.virtualenvs/shipp/lib/python2.7/site-packages/django/template/base.py" in render
123. return self._render(context)
File "/home/rob/.virtualenvs/shipp/lib/python2.7/site-packages/django/template/base.py" in _render
117. return self.nodelist.render(context)
File "/home/rob/.virtualenvs/shipp/lib/python2.7/site-packages/django/template/base.py" in render
744. bits.append(self.render_node(node, context))
File "/home/rob/.virtualenvs/shipp/lib/python2.7/site-packages/django/template/debug.py" in render_node
73. result = node.render(context)
File "/home/rob/.virtualenvs/shipp/lib/python2.7/site-packages/django/template/loader_tags.py" in render
127. return compiled_parent._render(context)
File "/home/rob/.virtualenvs/shipp/lib/python2.7/site-packages/django/template/base.py" in _render
117. return self.nodelist.render(context)
File "/home/rob/.virtualenvs/shipp/lib/python2.7/site-packages/django/template/base.py" in render
744. bits.append(self.render_node(node, context))
File "/home/rob/.virtualenvs/shipp/lib/python2.7/site-packages/django/template/debug.py" in render_node
73. result = node.render(context)
File "/home/rob/.virtualenvs/shipp/lib/python2.7/site-packages/django/template/loader_tags.py" in render
127. return compiled_parent._render(context)
File "/home/rob/.virtualenvs/shipp/lib/python2.7/site-packages/django/template/base.py" in _render
117. return self.nodelist.render(context)
File "/home/rob/.virtualenvs/shipp/lib/python2.7/site-packages/django/template/base.py" in render
744. bits.append(self.render_node(node, context))
File "/home/rob/.virtualenvs/shipp/lib/python2.7/site-packages/django/template/debug.py" in render_node
73. result = node.render(context)
File "/home/rob/.virtualenvs/shipp/lib/python2.7/site-packages/django/template/defaulttags.py" in render
311. return self.nodelist_true.render(context)
File "/home/rob/.virtualenvs/shipp/lib/python2.7/site-packages/django/template/base.py" in render
744. bits.append(self.render_node(node, context))
File "/home/rob/.virtualenvs/shipp/lib/python2.7/site-packages/django/template/debug.py" in render_node
73. result = node.render(context)
File "/home/rob/.virtualenvs/shipp/lib/python2.7/site-packages/django/template/defaulttags.py" in render
311. return self.nodelist_true.render(context)
File "/home/rob/.virtualenvs/shipp/lib/python2.7/site-packages/django/template/base.py" in render
744. bits.append(self.render_node(node, context))
File "/home/rob/.virtualenvs/shipp/lib/python2.7/site-packages/django/template/debug.py" in render_node
73. result = node.render(context)
File "/home/rob/.virtualenvs/shipp/lib/python2.7/site-packages/django/template/loader_tags.py" in render
64. result = block.nodelist.render(context)
File "/home/rob/.virtualenvs/shipp/lib/python2.7/site-packages/django/template/base.py" in render
744. bits.append(self.render_node(node, context))
File "/home/rob/.virtualenvs/shipp/lib/python2.7/site-packages/django/template/debug.py" in render_node
73. result = node.render(context)
File "/home/rob/.virtualenvs/shipp/lib/python2.7/site-packages/django/template/defaulttags.py" in render
437. url = reverse(view_name, args=args, kwargs=kwargs, current_app=context.current_app)
File "/home/rob/.virtualenvs/shipp/lib/python2.7/site-packages/django/core/urlresolvers.py" in reverse
391. *args, **kwargs)))
File "/home/rob/.virtualenvs/shipp/lib/python2.7/site-packages/django/core/urlresolvers.py" in reverse
312. possibilities = self.reverse_dict.getlist(lookup_view)
File "/home/rob/.virtualenvs/shipp/lib/python2.7/site-packages/django/core/urlresolvers.py" in _get_reverse_dict
229. self._populate()
File "/home/rob/.virtualenvs/shipp/lib/python2.7/site-packages/django/core/urlresolvers.py" in _populate
208. for name in pattern.reverse_dict:
File "/home/rob/.virtualenvs/shipp/lib/python2.7/site-packages/django/core/urlresolvers.py" in _get_reverse_dict
229. self._populate()
File "/home/rob/.virtualenvs/shipp/lib/python2.7/site-packages/django/core/urlresolvers.py" in _populate
208. for name in pattern.reverse_dict:
File "/home/rob/.virtualenvs/shipp/lib/python2.7/site-packages/django/core/urlresolvers.py" in _get_reverse_dict
229. self._populate()
File "/home/rob/.virtualenvs/shipp/lib/python2.7/site-packages/django/core/urlresolvers.py" in _populate
197. for pattern in reversed(self.url_patterns):
File "/home/rob/.virtualenvs/shipp/lib/python2.7/site-packages/django/core/urlresolvers.py" in _get_url_patterns
279. patterns = getattr(self.urlconf_module, "urlpatterns", self.urlconf_module)
File "/home/rob/.virtualenvs/shipp/lib/python2.7/site-packages/django/core/urlresolvers.py" in _get_urlconf_module
274. self._urlconf_module = import_module(self.urlconf_name)
File "/home/rob/.virtualenvs/shipp/lib/python2.7/site-packages/django/utils/importlib.py" in import_module
35. __import__(name)
File "/home/rob/projects/shipp/shipp/tickets/urls.py" in <module>
3. from shipp.tickets.views import TicketListView, TicketDetailView, \
File "/home/rob/projects/shipp/shipp/tickets/views.py" in <module>
15. from shipp.tickets.forms import TicketForm, TicketAttributesForm, \
File "/home/rob/projects/shipp/shipp/tickets/forms.py" in <module>
26. class TicketTimeSpentForm(forms.ModelForm):
File "/home/rob/.virtualenvs/shipp/lib/python2.7/site-packages/django/forms/models.py" in __new__
205. opts.exclude, opts.widgets, formfield_callback)
File "/home/rob/.virtualenvs/shipp/lib/python2.7/site-packages/django/forms/models.py" in fields_for_model
159. formfield = f.formfield(**kwargs)
File "/home/rob/projects/shipp/shipp/../shipp/tickets/fields.py" in formfield
43. return super(TimedeltaField, self).formfield(*args, **defaults)
Exception Type: TemplateSyntaxError at /admin/
Exception Value: Caught TypeError while rendering: Error when calling the metaclass bases
must be type, not None
トレースバックからわかるように、問題の原因となっている行（の43行目fields.py）は通常のsuper呼び出しです。このエラーが発生している理由について何か考えがありますか？上記の問題の原因となっているメソッドを削除すると、さまざまな場所でも発生します。