Flaskでpipenv --threeとすると'NoneType' object has no attribute 'pythons'とエラーがでます。

Question

前提・実現したいこと

「ゼロからFlsakがよくわかる本」をいう本で勉強しているのですが、その中で仮想環境を使う部分をやっています。
そこで、applicationというフォルダを作り、ターミナルでpipenv --threeとするとエラーがでます。

発生している問題・エラーメッセージ

Traceback (most recent call last):
  File "/Users/yudai/.pyenv/versions/3.6.5/bin/pipenv", line 8, in <module>
    sys.exit(cli())
  File "/Users/yudai/.pyenv/versions/3.6.5/lib/python3.6/site-packages/pipenv/vendor/click/core.py", line 764, in __call__
    return self.main(*args, **kwargs)
  File "/Users/yudai/.pyenv/versions/3.6.5/lib/python3.6/site-packages/pipenv/vendor/click/core.py", line 717, in main
    rv = self.invoke(ctx)
  File "/Users/yudai/.pyenv/versions/3.6.5/lib/python3.6/site-packages/pipenv/vendor/click/core.py", line 1114, in invoke
    return Command.invoke(self, ctx)
  File "/Users/yudai/.pyenv/versions/3.6.5/lib/python3.6/site-packages/pipenv/vendor/click/core.py", line 956, in invoke
    return ctx.invoke(self.callback, **ctx.params)
  File "/Users/yudai/.pyenv/versions/3.6.5/lib/python3.6/site-packages/pipenv/vendor/click/core.py", line 555, in invoke
    return callback(*args, **kwargs)
  File "/Users/yudai/.pyenv/versions/3.6.5/lib/python3.6/site-packages/pipenv/vendor/click/decorators.py", line 64, in new_func
    return ctx.invoke(f, obj, *args, **kwargs)
  File "/Users/yudai/.pyenv/versions/3.6.5/lib/python3.6/site-packages/pipenv/vendor/click/core.py", line 555, in invoke
    return callback(*args, **kwargs)
  File "/Users/yudai/.pyenv/versions/3.6.5/lib/python3.6/site-packages/pipenv/vendor/click/decorators.py", line 17, in new_func
    return f(get_current_context(), *args, **kwargs)
  File "/Users/yudai/.pyenv/versions/3.6.5/lib/python3.6/site-packages/pipenv/cli/command.py", line 208, in cli
    clear=state.clear,
  File "/Users/yudai/.pyenv/versions/3.6.5/lib/python3.6/site-packages/pipenv/core.py", line 574, in ensure_project
    pypi_mirror=pypi_mirror,
  File "/Users/yudai/.pyenv/versions/3.6.5/lib/python3.6/site-packages/pipenv/core.py", line 516, in ensure_virtualenv
    ensure_python(three=three, python=python)
  File "/Users/yudai/.pyenv/versions/3.6.5/lib/python3.6/site-packages/pipenv/core.py", line 397, in ensure_python
    path_to_python = find_a_system_python(python)
  File "/Users/yudai/.pyenv/versions/3.6.5/lib/python3.6/site-packages/pipenv/core.py", line 360, in find_a_system_python
    python_entry = finder.find_python_version(line)
  File "/Users/yudai/.pyenv/versions/3.6.5/lib/python3.6/site-packages/pipenv/vendor/pythonfinder/pythonfinder.py", line 113, in find_python_version
    return self.system_path.find_python_version(
  File "/Users/yudai/.pyenv/versions/3.6.5/lib/python3.6/site-packages/pipenv/vendor/pythonfinder/pythonfinder.py", line 54, in system_path
    ignore_unsupported=self.ignore_unsupported,
  File "/Users/yudai/.pyenv/versions/3.6.5/lib/python3.6/site-packages/pipenv/vendor/pythonfinder/models/path.py", line 451, in create
    ignore_unsupported=ignore_unsupported,
  File "<attrs generated init 75e45f144e3d5510d54dd5fca6730b98fa0220a9>", line 38, in __init__
  File "/Users/yudai/.pyenv/versions/3.6.5/lib/python3.6/site-packages/pipenv/vendor/pythonfinder/models/path.py", line 116, in __attrs_post_init__
    self._setup_pyenv()
  File "/Users/yudai/.pyenv/versions/3.6.5/lib/python3.6/site-packages/pipenv/vendor/pythonfinder/models/path.py", line 196, in _setup_pyenv
    version_glob_path="versions/*", ignore_unsupported=self.ignore_unsupported
  File "/Users/yudai/.pyenv/versions/3.6.5/lib/python3.6/site-packages/pipenv/vendor/pythonfinder/models/python.py", line 156, in create
    sort_function=sort_function, version_glob_path=version_glob_path)
  File "<attrs generated init 8b0973ebac28f7b68b6004369aca978b109647d6>", line 21, in __init__
  File "/Users/yudai/.pyenv/versions/3.6.5/lib/python3.6/site-packages/pipenv/vendor/pythonfinder/models/python.py", line 147, in get_pythons
    pythons.update(p.pythons)
AttributeError: 'NoneType' object has no attribute 'pythons'

試したこと

pythonsという属性を持っていませんと言われているような気がするのですが、何を言っているのかわかりません。
どなたかご指導お願いいたします。

補足情報（FW/ツールのバージョンなど）

Python 3.6.5
Flask 1.1.1
Werkzeug 0.16.0