eldoc-documentation-function

[martin rudalics]

When with Emacs 27 I insert in *scratch*

(defvar foo nil)

move point to the 'defvar' and do

(funcall eldoc-documentation-function)

I get something like

#("defvar: (SYMBOL &optional INITVALUE DOCSTRING)" 0 6 (face 
font-lock-keyword-face))

When I do the same with Emacs 28 I get

Debugger entered--Lisp error: (void-function nil)
  nil(:patient)
  eldoc--make-callback(:patient)
  eldoc-documentation-default()
  funcall(eldoc-documentation-default)
  (progn (funcall eldoc-documentation-function))
  eval((progn (funcall eldoc-documentation-function)) t)
  elisp--eval-last-sexp(nil)
  eval-last-sexp(nil)
  funcall-interactively(eval-last-sexp nil)
  call-interactively(eval-last-sexp nil nil)
  command-execute(eval-last-sexp)

What do I have to do to get the Emacs 27 behavior with Emacs 28?

Thanks in advance, martin