Unexpected error using org-gnus-follow-link

["Fraga, Eric" <e.fraga@ucl.ac.uk>]

Hello all,

I upgraded both Emacs (from git) and org (elpa) in the last couple of
days.  Today I find that I get the following error using
org-open-at-point:

--8<---------------cut here---------------start------------->8---
Debugger entered--Lisp error: (void-function nil)
  nil()
  org-gnus-follow-link("nnml+outlook:mail.dt2022" "[...]")
  org-gnus-open("nnml+outlook:mail.dt2022#[...]" nil)
  org-link-open((link (:standard-properties [1994 nil 2111 2179 2181 0 nil nil nil nil nil nil nil nil #<buffer todo.org> nil nil (paragraph (:standard-properties [1994 1994 1994 2182 2182 0 nil nil element t (3 . 1815) nil nil nil #<buffer todo.org> nil nil (item ...)]))] :type "gnus" :type-explicit-p t :path "nnml+outlook:mail.dt2022#[...]" :format bracket :raw-link "gnus:nnml+outlook:mail.dt2022#[...]" :application nil :search-option nil)) nil)
  org-open-at-point(nil)
  funcall-interactively(org-open-at-point nil)
  command-execute(org-open-at-point)
--8<---------------cut here---------------end--------------->8---

with message reference elided.

I cannot really reduce this to a minimal example or use -Q due to need
to have gnus configured.

Any suggestions on how to resolve this?  The vast majority of my TODO
entries have gnus email links and they have always worked just fine.
The problem could, of course, be with gnus but I cannot tell from the
above backtrace.

Thank you,
eric
-- 
: Eric S Fraga, with org 9.7.17-3ae179 in Emacs 31.0.50