Environment

• pip version: pip 19.2.1 from /home/dinko/.local/lib/python3.6/site-packages/pip (python 3.6)
• Python version: Python 3.6.8
• OS: 18.04.1-Ubuntu x86_64

Description
I tried to set some pip config variables for pip to use, but I'm unable to unset some that contain underscores.

Expected behavior
pip config variables should be unset when used like pip config unset global.variable_name

How to Reproduce

1. pip config set global.index-url example.com
2. pip config list (global.index-url='example.com' listed)
3. pip config unset global.index-url

Output

ERROR: Fatal Internal error [id=1]. Please report as a bug.

NOTE: I created some more variables like global.a-b-c and global.a-b-c-d but I'm able to unset some, while some return error like ERROR: Fatal Internal error [id=1]. Please report as a bug.