>>>>> Eli Zaretskii writes:

    > What do you mean by "isn't a directory"?  FILE is supposed to be a
    > documentation file, not a directory, see the doc string of this
    > function.  Do you mean that FILE is a relative file name so that
    > file-name-directory returns nil for it?  If so, I think the
    > correct fix would be to reverse the order:

    >   (docs-directory (file-name-directory (expand-file-name file)))

Yeah, you're completely right, this was a misunderstanding on my part;
my fix just happened to also work so I made some incorrect assumptions.

After some further testing, this seems to happen for packages for which
the doc file is in the base directory of the package (which causes
'file-name-directory' to return nil as the doc file name is relative to
the package dir).

I attached an updated patch.

    > Can you show a recipe to reproduce this problem, preferably
    > starting from "emacs -Q"?

1. $ emacs -Q
2. M-x package-vc-install RET eat RET
3. A few seconds later,
   "package-vc--unpack-1: Wrong type argument: stringp, nil"

    > And finally, this issue exists on the emacs-29 release branch as
    > well, doesn't it?

I haven't tested it, but I don't see a reason it wont.