For us who run emacs-snapshot, this bug pops up in today's emacs-snapshot
20130410-1. A simple work around seems to be to load these lines:
(defun gnus-nnir-group-p (group)
"Say whether GROUP is nnir or not."
(if (gnus-group-prefixed-p group)
(eq 'nnir (car (gnus-find-method-for-group group)))
(and group (string-match "^nnir" group))))
found in emacs-snapshot's /usr/share/emacs/24.3.50/lisp/gnus/nnir.el.gz
Gijs
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