Getting `help-xref-button: Wrong type argument: symbolp, (lambda ...)' when using `describe-key' with `describe-function-1' in `help+20'

Question

When using describe-key, the following block from the help+20 package, in the describe-function-1 function:

(help-xref-button 1 #'(lambda (arg)
                      (let ((location
                         (find-function-noselect arg)))
                    (pop-to-buffer (car location))
                    (goto-char (cdr location))))
                function)

Is causing the following error:

help-xref-button: Wrong type argument: symbolp, (lambda (arg) (let ((location (find-function-noselect arg))) (pop-to-buffer (car location)) (goto-char (cdr location))))

I tried pulling the lambda out into its own function:

;;;###autoload
(defun describe-function-1--hyperlink (arg)
    (let ((location
        (find-function-noselect arg)))
  (pop-to-buffer (car location))
  (goto-char (cdr location))))

Modifying the help-xref-button call accordingly:

(help-xref-button 1 #'describe-function-1--hyperlink function)

But am getting the following error:

make-text-button: Unknown button type ‘describe-function-1--hyperlink’

How can I create a new button type, if that's what I really need to do?

Answer 1

As mentioned by NickD and Drew in the comments of the question, the issue was my using help+20, which is used only for emacs 20, instead of help+.