WARNING: Keyring is skipped due to an exception: 'NoneType' object has no attribute 'endswith'

[ffissore]

When trying to install a package that's listed in my artifact registry, I get the warning

WARNING: Keyring is skipped due to an exception: 'NoneType' object has no attribute 'endswith'
ERROR: Could not find a version that satisfies the requirement <PACKAGE NAME>
ERROR: No matching distribution found for <PACKAGE NAME>

and the package is not installed

This seems to be due to my pip.conf mentioning the artifact registry as a extra-index-url, rather than index-url
If I install the same package with pip install --index-url https://... <PACKAGE NAME> it works as expected