jupyterhubでsignup画面が出ない（500 : Internal Server Error）

前提・実現したいこと

実行マシン：centOS7

Nativeauthenticatorを用いたjupyterhubを使用したい。
docker-composeを使用している。

発生している問題・エラーメッセージ

jupyterhub_1  | [E 2020-12-23 00:38:37.495 JupyterHub web:1789] Uncaught exception GET /hub/signup (::ffff:xxx.xxx.xxx.xxx)
jupyterhub_1  |     HTTPServerRequest(protocol='http', host='localhost:8000', method='GET', uri='/hub/signup', version='HTTP/1.1', remote_ip='::ffff:xxx.xxx.xxx.xxx')
jupyterhub_1  |     Traceback (most recent call last):
jupyterhub_1  |       File "/usr/local/lib/python3.8/dist-packages/tornado/web.py", line 1704, in _execute
jupyterhub_1  |         result = await result
jupyterhub_1  |       File "/usr/local/lib/python3.8/dist-packages/nativeauthenticator/handlers.py", line 41, in get
jupyterhub_1  |         self.finish(html)
jupyterhub_1  |       File "/usr/local/lib/python3.8/dist-packages/jupyterhub/handlers/base.py", line 172, in finish
jupyterhub_1  |         super().finish(*args, **kwargs)
jupyterhub_1  |       File "/usr/local/lib/python3.8/dist-packages/tornado/web.py", line 1127, in finish
jupyterhub_1  |         self.write(chunk)
jupyterhub_1  |       File "/usr/local/lib/python3.8/dist-packages/tornado/web.py", line 841, in write
jupyterhub_1  |         raise TypeError(message)
jupyterhub_1  |     TypeError: write() only accepts bytes, unicode, and dict objects
jupyterhub_1  |
jupyterhub_1  | [E 2020-12-23 00:38:37.512 JupyterHub log:173] {
jupyterhub_1  |       "X-Forwarded-Host": "localhost:8000",
jupyterhub_1  |       "X-Forwarded-Proto": "http",
jupyterhub_1  |       "X-Forwarded-Port": "8000",
jupyterhub_1  |       "X-Forwarded-For": "::ffff:xxx.xxx.xxx.xxx",
jupyterhub_1  |       "Accept-Language": "ja-JP,ja;q=0.9,en-US;q=0.8,en;q=0.7",
jupyterhub_1  |       "Accept-Encoding": "gzip, deflate",
jupyterhub_1  |       "Referer": "http://localhost:8000/hub/login?next=%2Fhub%2F",
jupyterhub_1  |       "Accept": "text/html,application/xhtml+xml,application/xml;q=0.9,image/avif,image/webp,image/apng,*/*;q=0.8,application/signed-exchange;v=b3;q=0.9",
jupyterhub_1  |       "User-Agent": "Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/87.0.4280.88 Safari/537.36",
jupyterhub_1  |       "Upgrade-Insecure-Requests": "1",
jupyterhub_1  |       "Connection": "close",
jupyterhub_1  |       "Host": "localhost:8000"
jupyterhub_1  |     }
jupyterhub_1  | [E 2020-12-23 00:38:37.512 JupyterHub log:181] 500 GET /hub/signup (@::ffff:xxx.xxx.xxx.xxx) 19.67ms

試したこと

調べてみたが、TypeError: write() only accepts bytes, unicode, and dict objectsについてなかなか出てこなかった。
tornadoが怪しいみたいだったので、インストールしたが駄目だった。
jupyterhub_config.pyに追加したもの抜粋

c.NativeAuthenticator.open_signup = True
c.JupyterHub.authenticator_class = 'nativeauthenticator.NativeAuthenticator'
c.Authenticator.admin_users = {'user'}
c.JupyterHub.hub_port = 8081
c.JupyterHub.spawner_class='dockerspawner.DockerSpawner'