'graph' argument in pipenv breaks it

I discovered this by setting up pipenv on the latest Pycharm Professional. Once I wanted to install some 3rd party libraries with it, I got the following:

/Users/<myuser>/Library/Python/2.7/bin/pipenv graph --json
    Traceback (most recent call last):
      File "/Users/<myuser>/Library/Python/2.7/bin/pipenv", line 11, in <module>
        sys.exit(cli())
      File "/Users/<myuser>/Library/Python/2.7/lib/python/site-packages/pipenv/vendor/click/core.py", line 764, in __call__
        return self.main(*args, **kwargs)
      File "/Users/<myuser>/Library/Python/2.7/lib/python/site-packages/pipenv/vendor/click/core.py", line 717, in main
        rv = self.invoke(ctx)
      File "/Users/<myuser>/Library/Python/2.7/lib/python/site-packages/pipenv/vendor/click/core.py", line 1137, in invoke
        return _process_result(sub_ctx.command.invoke(sub_ctx))
      File "/Users/<myuser>/Library/Python/2.7/lib/python/site-packages/pipenv/vendor/click/core.py", line 956, in invoke
        return ctx.invoke(self.callback, **ctx.params)
      File "/Users/<myuser>/Library/Python/2.7/lib/python/site-packages/pipenv/vendor/click/core.py", line 555, in invoke
        return callback(*args, **kwargs)
      File "/Users/<myuser>/Library/Python/2.7/lib/python/site-packages/pipenv/cli/command.py", line 528, in graph
        do_graph(bare=bare, json=json, json_tree=json_tree, reverse=reverse)
      File "/Users/<myuser>/Library/Python/2.7/lib/python/site-packages/pipenv/core.py", line 2397, in do_graph
        for d in simplejson.loads(c.out):
      File "/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/json/__init__.py", line 338, in loads
        return _default_decoder.decode(s)
      File "/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/json/decoder.py", line 366, in decode
        obj, end = self.raw_decode(s, idx=_w(s, 0).end())
      File "/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/json/decoder.py", line 384, in raw_decode
        raise ValueError("No JSON object could be decoded")
    ValueError: No JSON object could be decoded

I am on MacOS High Sierra and installed pipenv via pip. I tried running the command without the --json argument and got:

ERROR:  Traceback (most recent call last):
  File "/Users/<user>/Library/Python/2.7/lib/python/site-packages/pipenv/vendor/pipdeptree.py", line 16, in <module>
    from pipenv.vendor.pip_shims import get_installed_distributions, FrozenRequirement
ImportError: No module named pipenv.vendor.pip_shims

I don't know what the problem is as google isn't turning up anything useful.

Here's some facts:

> pip show pipenv
Name: pipenv
Version: 2018.10.9
Summary: Python Development Workflow for Humans.
Home-page: https://github.com/pypa/pipenv
Author: Kenneth Reitz
Author-email: me@kennethreitz.org
License: MIT
Location: /Users/<user>/Library/Python/2.7/lib/python/site-packages
Requires: virtualenv-clone, enum34, typing, pip, certifi, virtualenv, setuptools
Required-by: