Problem with pip in PyCharm settings dialog

When trying to install modules from PyCharm's settings dialog (Linux), I get the following error...

Error: Traceback (most recent call last):
  File "/home/eric/Desktop/pycharm-2017.3.4/helpers/packaging_tool.py", line 106, in do_install
    import pip
  File "/usr/lib/python3/dist-packages/pip/__init__.py", line 14, in <module>
    from pip.utils import get_installed_distributions, get_prog
  File "/usr/lib/python3/dist-packages/pip/utils/__init__.py", line 30, in <module>
    from pip._vendor.retrying import retry
ImportError: No module named 'pip._vendor.retrying'

It sounds like a pip problem, but I'm not sure how to fix it.  How do I resolve this?

Thanks

Eric