ERROR: File not found! Please provide a valid filename as an argument.

Overview

https://gitlab.com/gitlab-org/gitlab-development-kit/-/jobs/2006345712#L28 has errors, despite passing CI:

--snip--
$ make lint
Vale: ✔ 0 errors, 0 warnings and 0 suggestions in 66 files.
MarkdownLint: OK
Check internal links: 
ERROR: File not found! Please provide a valid filename as an argument.

cc @eread