In my case, common-parent-directory is nil.

> When I call C-x p d (project-find-dir), it reports the error message:
> `project--read-file-cpd-relative: Wrong type argument: stringp, nil`
>
> Finaly I found it is due the the zero length of
> `common-parent-directory` in `project--read-file-cpd-relative` as I
> using `fd` to provide the list of dirs which does not include the common
> project root in the returnd dirs.
>
> In `project--read-file-cpd-relative`, the zero length of
> `common-parent-directory` is handled for `prompt` but not for `abbr-cpd`
> which will be used as parameter of `string-prefix-p`.
>
> Below is the diff and a working patch. But no sure for the impact of HIST.
>
> ```
> diff --git a/lisp/progmodes/project.el b/lisp/progmodes/project.el
> index 559da6dd649..8c4ea8eeca0 100644
> --- a/lisp/progmodes/project.el
> +++ b/lisp/progmodes/project.el
> @@ -1039,7 +1039,9 @@ project--read-file-cpd-relative
> (_ (when included-cpd
> (setq substrings (cons "./" substrings))))
> (new-collection (project--file-completion-table substrings))
> - (abbr-cpd (abbreviate-file-name common-parent-directory))
> + (abbr-cpd (if (zerop cpd-length)
> + ""
> + (abbreviate-file-name common-parent-directory)))

Strange, I can't reproduce the problem. When cpd-length is 0,
then for the empty string (abbreviate-file-name "") returns "".