Problems with nrss-url-field

[Desmond Rivet]

Hi all,

I have the following in my .gnus.el, cribbed from some website I've
forgotten:

(require 'browse-url)
(defun browse-nnrss-url( arg )
  (interactive "p")
  (let ((url (assq nnrss-url-field
                  (mail-header-extra
                   (gnus-data-header
                    (assq (gnus-summary-article-number)
                          gnus-newsgroup-data))))))
   (if url
       (progn
         (browse-url (cdr url))
         (gnus-summary-mark-as-read-forward 1))
     (gnus-summary-scroll-up arg))))

(eval-after-load "gnus"
  #'(define-key gnus-summary-mode-map
     (kbd "C-c <RET>") 'browse-nnrss-url))
(add-to-list 'nnmail-extra-headers nnrss-url-field)

The idea is that I want C-c <RET> to launch the RSS url in a browser.

This actually worked fine the first couple of times I started up Gnus.
But now when I start up Gnus, I get this in the minibuffer:

Error in ~/.gnus: nnrss-url-field

Commenting out the elisp code above allows Gnus to start up.

My elisp is...sketchy, but I can generally follow snippets of code
when I put in the effort.  But I don't even know how to debug this...

Any ideas?  Even pointers on how to get more information on what's
going wrong would be helpful...

-- 
Desmond Rivet

Pain is weakness leaving the body.