title: AttributeError: 'NoneType' object has no attribute 'excluded_of'
body: <p>I am getting attribute error while installing the dependencies via pip</p>
  <pre><code>Traceback (most recent call last):
    File &quot;/home/jpg/.virtual_env/cloud/lib/python3.6/site-packages/pip/_internal/cli/base_command.py&quot;, line 210, in _main
      status = self.run(options, args)
    File &quot;/home/jpg/.virtual_env/cloud/lib/python3.6/site-packages/pip/_internal/cli/req_command.py&quot;, line 180, in wrapper
      return func(self, options, args)
    File &quot;/home/jpg/.virtual_env/cloud/lib/python3.6/site-packages/pip/_internal/commands/install.py&quot;, line 319, in run
      reqs, check_supported_wheels=not options.target_dir
    File &quot;/home/jpg/.virtual_env/cloud/lib/python3.6/site-packages/pip/_internal/resolution/resolvelib/resolver.py&quot;, line 122, in resolve
      requirements, max_rounds=try_to_avoid_resolution_too_deep,
    File &quot;/home/jpg/.virtual_env/cloud/lib/python3.6/site-packages/pip/_vendor/resolvelib/resolvers.py&quot;, line 445, in resolve
      state = resolution.resolve(requirements, max_rounds=max_rounds)
    File &quot;/home/jpg/.virtual_env/cloud/lib/python3.6/site-packages/pip/_vendor/resolvelib/resolvers.py&quot;, line 344, in resolve
      success = self._backtrack()
    File &quot;/home/jpg/.virtual_env/cloud/lib/python3.6/site-packages/pip/_vendor/resolvelib/resolvers.py&quot;, line 287, in _backtrack
      criterion = self.state.criteria[name].excluded_of([candidate])
  AttributeError: 'NoneType' object has no attribute 'excluded_of'
  </code></pre>
  <p>My <code>requirements.txt</code> looks like as</p>
  <pre><code>celery==5.0.2
  billiard&lt;4.0,&gt;=3.6.0
  redis==3.5.3
  redis-log-handler==0.0.1.dev32
  </code></pre>