Thanks Michael. On further investigation, I've found that this minimal code
(after starting with emacs -Q) produces the same unexpected error:

(condition-case nil
(emacs-repository-get-branch nil)
  (error nil))

I don't think I can investigate further as condition-case is not a lisp
function.

Kind regards

Geoff

On Thu, 15 Nov 2018 at 11:40, Michael Albinus <michael.albinus@gmx.de>
wrote:

> Geoffrey Ferrari <geoffrey.ferrari@oriel.oxon.org> writes:
>
> Hi Geoffrey,
>
> > The issue can be traced to the definition of tramp-repository-branch
> > in
> > trampver.el. This calls:
> >
> > (emacs-repository-get-branch
> > (locate-dominating-file (locate-library "tramp") ".git"))
> >
> > However, (locate-dominating-file (locate-library "tramp") ".git")
> > evaluates to nil, meaning that above is equivalent to
> > (emacs-repository-get-branch nil).
> >
> > This causes the function emacs-repository-get-branch (in version.el)
> > to
> > rely on the variable source-directory i.e. which references the source
> > directory where the emacs
> > binary was built, which of course does not exist on an end user's
> > system.
>
> Yes. However, the initial value of tramp-repository-branch is
>
> --8<---------------cut here---------------start------------->8---
>   (ignore-errors
>     ;; Suppress message from `emacs-repository-get-branch'.
>     (let ((inhibit-message t))
>       ;; `emacs-repository-get-branch' has been introduced with Emacs 27.1.
>       (with-no-warnings
>         (emacs-repository-get-branch
>          (locate-dominating-file (locate-library "tramp") ".git")))))
> --8<---------------cut here---------------end--------------->8---
>
> I'm wondering why you see an error, although the code is wrapped by
> `ignore-errors'.
>
> Best regards, Michael.
>