title: Failing to initialize pipenv in mac (TypeError: sequence item 0: expected str instance, NoneType found)
body: <p>I'm a Mac user, and I frequently face frustrations when installing python packages. </p>
  
  <p>This time, pipenv seems not working properly.
  I successfully installed pipenv.</p>
  
  <pre><code>$pipenv --version
  pipenv, version 11.10.1
  </code></pre>
  
  <p>However, When I try to init pipenv shell, I got error like this:</p>
  
  <pre><code>Creating a virtualenv for this project…
      Using /Library/Frameworks/Python.framework/Versions/3.6/bin/python3.6 (3.6.4) to create virtualenv…
      ⠋Already using interpreter /Library/Frameworks/Python.framework/Versions/3.6/bin/python3.6
      Using base prefix '/Library/Frameworks/Python.framework/Versions/3.6'
      Traceback (most recent call last):
        File "/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/runpy.py", line 193, in _run_module_as_main
          "__main__", mod_spec)
        File "/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/runpy.py", line 85, in _run_code
          exec(code, run_globals)
        File "/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/site-packages/virtualenv.py", line 2349, in &lt;module&gt;
          main()
        File "/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/site-packages/virtualenv.py", line 712, in main
          symlink=options.symlink)
        File "/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/site-packages/virtualenv.py", line 927, in create_environment
          site_packages=site_packages, clear=clear, symlink=symlink))
        File "/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/site-packages/virtualenv.py", line 1140, in install_python
          copy_required_modules(home_dir, symlink)
        File "/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/site-packages/virtualenv.py", line 1070, in copy_required_modules
          dst_filename = change_prefix(filename, dst_prefix)
        File "/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/site-packages/virtualenv.py", line 1044, in change_prefix
          (filename, prefixes)
      AssertionError: Filename /Users/ChaewonKong/Desktop/dev/re.py does not start with any of these prefixes: ['/Library/Frameworks/Python.framework/Versions/3.6/Extras/lib/python', '/Users/ChaewonKong/Library/Python/3.6/lib/python/site-packages', '/Users/ChaewonKong/.local/lib/python/3.6/site-packages', '/Users/ChaewonKong/Library/Python/3.6/site-packages', '/Library/Frameworks/Python.framework/Versions/3.6', '/Library/Frameworks/Python.framework/Versions/3.6', '/Library/Python/3.6/site-packages']
  
      Virtualenv location: 
      Creating a Pipfile for this project…
      Traceback (most recent call last):
        File "/Library/Frameworks/Python.framework/Versions/3.6/bin/pipenv", line 11, in &lt;module&gt;
          sys.exit(cli())
        File "/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/site-packages/pipenv/vendor/click/core.py", line 722, in __call__
          return self.main(*args, **kwargs)
        File "/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/site-packages/pipenv/vendor/click/core.py", line 697, in main
          rv = self.invoke(ctx)
        File "/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/site-packages/pipenv/vendor/click/core.py", line 1066, in invoke
          return _process_result(sub_ctx.command.invoke(sub_ctx))
        File "/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/site-packages/pipenv/vendor/click/core.py", line 895, in invoke
          return ctx.invoke(self.callback, **ctx.params)
        File "/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/site-packages/pipenv/vendor/click/core.py", line 535, in invoke
          return callback(*args, **kwargs)
        File "/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/site-packages/pipenv/cli.py", line 607, in shell
          three=three, python=python, fancy=fancy, shell_args=shell_args
        File "/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/site-packages/pipenv/core.py", line 2149, in do_shell
          ensure_project(three=three, python=python, validate=False)
        File "/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/site-packages/pipenv/core.py", line 648, in ensure_project
          ensure_pipfile(validate=validate, skip_requirements=skip_requirements, system=system)
        File "/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/site-packages/pipenv/core.py", line 302, in ensure_pipfile
          project.create_pipfile(python=python)
        File "/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/site-packages/pipenv/project.py", line 588, in create_pipfile
          required_python = self.which('python')
        File "/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/site-packages/pipenv/core.py", line 126, in which
          p = os.sep.join([location] + ['bin/{0}'.format(command)])
      TypeError: sequence item 0: expected str instance, NoneType found
  </code></pre>