1 files changed, 3 insertions, 3 deletions

diff --git a/lisp/gnus/gnus-sum.el b/lisp/gnus/gnus-sum.el
index 694e70a640a..9c751cd19d7 100644
--- a/lisp/gnus/gnus-sum.el
+++ b/lisp/gnus/gnus-sum.el
@@ -136,9 +136,9 @@ which only takes Subjects into consideration; and
 `gnus-gather-threads-by-references', which compared the References
 headers of the articles to find matches."
   :group 'gnus-thread
-  :type '(set (function-item gnus-gather-threads-by-subject)
-              (function-item gnus-gather-threads-by-references)
-              (function :tag "other")))
+  :type '(radio (function-item gnus-gather-threads-by-subject)
+                (function-item gnus-gather-threads-by-references)
+                (function :tag "other")))
 
 ;; Added by Per Abrahamsen <amanda@iesd.auc.dk>.
 (defcustom gnus-summary-same-subject ""