python-buildpack: Could not install pipenv: exit status 1

What version of Cloud Foundry and CF CLI are you using? (i.e. What is the output of running cf curl /v2/info && cf version?

{
   "name": "",
   "build": "",
   "support": "",
   "version": 0,
   "description": "",
   "authorization_endpoint": "https://login.fr.cloud.gov",
   "token_endpoint": "https://uaa.fr.cloud.gov",
   "min_cli_version": null,
   "min_recommended_cli_version": null,
   "api_version": "2.106.0",
   "app_ssh_endpoint": "ssh.fr.cloud.gov:2222",
   "app_ssh_host_key_fingerprint": "fd:99:34:45:f5:b8:3f:fc:bf:a0:67:cd:ba:10:75:52",
   "app_ssh_oauth_client": "ssh-proxy",
   "doppler_logging_endpoint": "wss://doppler.fr.cloud.gov:443"
}
cf version 6.34.1+bbdf81482.2018-01-17

What version of the buildpack you are using?

-----> Python Buildpack version 1.6.11

If you were attempting to accomplish a task, what was it you were attempting to do?

cf push

What did you expect to happen?

Application deployed successfully.

What was the actual behavior?

# ... removed for brevity / security
-----> Installing pipenv 11.1.4
       Download [https://buildpacks.cloudfoundry.org/dependencies/pipenv/pipenv-v11.1.4-7394418e.tgz]
       Generating 'requirements.txt' with pipenv
Traceback (most recent call last):
  File "/tmp/contents126611305/deps/0/bin/pipenv", line 11, in <module>
    load_entry_point('pipenv==11.1.4', 'console_scripts', 'pipenv')()
  File "/tmp/contents126611305/deps/0/python/lib/python3.6/site-packages/pipenv/vendor/click/core.py", line 722, in __call__
    return self.main(*args, **kwargs)
  File "/tmp/contents126611305/deps/0/python/lib/python3.6/site-packages/pipenv/vendor/click/core.py", line 697, in main
    rv = self.invoke(ctx)
  File "/tmp/contents126611305/deps/0/python/lib/python3.6/site-packages/pipenv/vendor/click/core.py", line 1066, in invoke
    return _process_result(sub_ctx.command.invoke(sub_ctx))
  File "/tmp/contents126611305/deps/0/python/lib/python3.6/site-packages/pipenv/vendor/click/core.py", line 895, in invoke
    return ctx.invoke(self.callback, **ctx.params)
  File "/tmp/contents126611305/deps/0/python/lib/python3.6/site-packages/pipenv/vendor/click/core.py", line 535, in invoke
    return callback(*args, **kwargs)
  File "/tmp/contents126611305/deps/0/python/lib/python3.6/site-packages/pipenv/cli.py", line 238, in lock
    core.ensure_project(three=three, python=python)
  File "/tmp/contents126611305/deps/0/python/lib/python3.6/site-packages/pipenv/core.py", line 638, in ensure_project
    crayons.green(shorten_path(path_to_python))
TypeError: __str__ returned non-string (type NoneType)
       **ERROR** Could not install pipenv: exit status 1
Failed to compile droplet: Failed to run all supply scripts: exit status 14
# ... removed for brevity / security

Please confirm where necessary:

• I have included a log output
• My log includes an error message
• I have included steps for reproduction