我从djangorestframework的JSONEncoder类中收到以下异常：
Traceback:
File "/home/ubuntu/Envs/env_md/lib/python3.6/site-packages/django/core/handlers/exception.py" in inner
34. response = get_response(request)
File "/home/ubuntu/Envs/env_md/lib/python3.6/site-packages/django/core/handlers/base.py" in _get_response
156. response = self.process_exception_by_middleware(e, request)
File "/home/ubuntu/Envs/env_md/lib/python3.6/site-packages/django/core/handlers/base.py" in _get_response
154. response = response.render()
File "/home/ubuntu/Envs/env_md/lib/python3.6/site-packages/django/template/response.py" in render
106. self.content = self.rendered_content
File "/home/ubuntu/Envs/env_md/lib/python3.6/site-packages/rest_framework/response.py" in rendered_content
72. ret = renderer.render(self.data, accepted_media_type, context)
File "/home/ubuntu/Envs/env_md/lib/python3.6/site-packages/rest_framework/renderers.py" in render
106. allow_nan=not self.strict, separators=separators
File "/home/ubuntu/Envs/env_md/lib/python3.6/site-packages/rest_framework/utils/json.py" in dumps
28. return json.dumps(*args, **kwargs)
File "/usr/lib/python3.6/json/__init__.py" in dumps
238. **kw).encode(obj)
File "/usr/lib/python3.6/json/encoder.py" in encode
199. chunks = self.iterencode(o, _one_shot=True)
File "/usr/lib/python3.6/json/encoder.py" in iterencode
257. return _iterencode(o, 0)
File "/home/ubuntu/Envs/env_md/lib/python3.6/site-packages/rest_framework/utils/encoders.py" in default
56. elif (coreapi is not None) and isinstance(obj, (coreapi.Document, coreapi.Error)):
Exception Type: AttributeError at /v0/ui/website/subject-assignment/
Exception Value: module 'coreapi' has no attribute 'Document'
因此，基本上，这就是说我的应用程序（coreapi）没有Document属性。所有其他api都可以正常工作，但是在一个特定的api中却出现此错误。我也找不到与该API有任何不同的地方。 djangorestframework有什么问题吗？或者我应该怎么做调试？
注意：我使用的是django 2.1.4，djangorestframework == 3.9.0，而python版本是3.6.5。