Environment

• pip version: 20.3.3
• Python version: 3.9.0
• OS: OS/X 10.15.7

I'm using pyenv. I'm not an active Python developer, so please bear with me if I'm missing important details.

Description
I try to remove the "global.extra-index-url" key from the pip configuration by:

`pip config unset global.extra-index-url`

Which then responses with:

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

Expected behavior

I would expect to either remove the key or to prompt an error message that I could use to investigate into the reason of the failing of the command.

How to Reproduce
Sorry, I don't have any clue, how this could be reproduced.

Output

% pip config unset global.extra-index-url 
ERROR: Fatal Internal error [id=1]. Please report as a bug.

Maybe the current value of the configuration gives a clue (some values slightly changed):

% pip config list
global.extra-index-url='\nhttps://torsten:[email protected]/customer/api/pypi/    https://customer.jfrog.io/customer/api/pypi/customer-pip3-stable/simple\nhttps://pypi.python.org/simple/'

There was a similar issue reported last year: #6940 Unfortunately, the issue was closed without giving and hint to how it was resolved.