500 error while trying to delete feed.

[tribela]

I tried to delete one feed. but Earth Reader crashed.

Here's the traceback

Traceback (most recent call last):
  File "/Users/kjwon15/Library/Python/2.7/lib/python/site-packages/flask/app.py", line 1836, in __call__
    return self.wsgi_app(environ, start_response)
  File "build/bdist.macosx-10.9-intel/egg/sassutils/wsgi.py", line 100, in __call__
    return self.app(environ, start_response)
  File "/Users/kjwon15/Desktop/git/earthreader-web/earthreader/web/wsgi.py", line 59, in __call__
    return self.app(environ, start_response)
  File "/Users/kjwon15/Library/Python/2.7/lib/python/site-packages/flask/app.py", line 1820, in wsgi_app
    response = self.make_response(self.handle_exception(e))
  File "/Users/kjwon15/Library/Python/2.7/lib/python/site-packages/flask/app.py", line 1403, in handle_exception
    reraise(exc_type, exc_value, tb)
  File "/Users/kjwon15/Library/Python/2.7/lib/python/site-packages/flask/app.py", line 1817, in wsgi_app
    response = self.full_dispatch_request()
  File "/Users/kjwon15/Library/Python/2.7/lib/python/site-packages/flask/app.py", line 1477, in full_dispatch_request
    rv = self.handle_user_exception(e)
  File "/Users/kjwon15/Library/Python/2.7/lib/python/site-packages/flask/app.py", line 1381, in handle_user_exception
    reraise(exc_type, exc_value, tb)
  File "/Users/kjwon15/Library/Python/2.7/lib/python/site-packages/flask/app.py", line 1475, in full_dispatch_request
    rv = self.dispatch_request()
  File "/Users/kjwon15/Library/Python/2.7/lib/python/site-packages/flask/app.py", line 1461, in dispatch_request
    return self.view_functions[rule.endpoint](**req.view_args)
  File "/Users/kjwon15/Desktop/git/earthreader-web/earthreader/web/__init__.py", line 275, in delete_feed
    cursor.discard(target)
  File "/Users/kjwon15/Desktop/git/libearth/libearth/subscribe.py", line 188, in discard
    if deleted_at <= outline.created_at:
TypeError: can't compare datetime.datetime to NoneType