(or alternatively just binding buffer-file-name to nil after default-directory).

but it feels like the fix should happen in vc.el, by being able to
communicate that we the file set should be derived from the default
directory, and the current file should be ignored...

> In a minimal environment with emacs -Q, the following steps can
> reproduce my issue:
>
>
> ;; a simple package by purcell
> (package-vc-install "https://github.com/purcell/unfill")
> ;; then try to upgrade it in a buffer that buffer-file-name is not nil
> ;; and the file is not under Git version control system.
> M-x package-vc-upgrade unfill RET
> ;; now the error may occurs
>
>
> Regards
>
>
> YI YUE

-- 
	Philip Kaludercic on peregrine