diff --git a/lisp/mail/rmail.el b/lisp/mail/rmail.el
index 03be8dce49f..6b9ea1b019d 100644
--- a/lisp/mail/rmail.el
+++ b/lisp/mail/rmail.el
@@ -659,6 +659,7 @@ Note:    it means the file has no messages in it.\n\^_")))
       (setq coding-system (detect-coding-region from to t)))
     (unless (memq coding-system
 		  '(undecided undecided-unix))
+      (set-buffer-modified-p t)		; avoid locking when decoding
       (decode-coding-region from to coding-system)
       (setq coding-system last-coding-system-used))
     (set-buffer-modified-p modifiedp)