diff --git a/lisp/mail/rmail.el b/lisp/mail/rmail.el
index c598e67dab8..49eaeb560e0 100644
--- a/lisp/mail/rmail.el
+++ b/lisp/mail/rmail.el
@@ -3356,12 +3356,12 @@ removing prefixes such as Re:, Fwd: and so on and mailing list
 tags such as [tag]."
   (let ((subject (or (rmail-get-header "Subject" msgnum) ""))
 	(regexp "\\`[ \t\n]*\\(\\(\\w\\{1,4\\}\u00a0*[:：]\\|\\[[^]]+]\\)[ \t\n]+\\)*"))
+    (setq subject (rfc2047-decode-string subject))
     ;; Corporate mailing systems sometimes add `[External] :'; if that happened,
     ;; delete everything up thru there.  Empirically, that deletion makes
     ;; the Subject match the other messages in the thread.
     (if (string-match "\\[external][ \t\n]*:" subject)
         (setq subject (substring subject (match-end 0))))
-    (setq subject (rfc2047-decode-string subject))
     (setq subject (replace-regexp-in-string regexp "" subject))
     (replace-regexp-in-string "[ \t\n]+" " " subject)))