diff --git a/lisp/international/mule-util.el b/lisp/international/mule-util.el
index 7c51409422b..8ac56b4bd65 100644
--- a/lisp/international/mule-util.el
+++ b/lisp/international/mule-util.el
@@ -327,6 +327,7 @@ coding systems ordered by priority."
 	 (mapc (function (lambda (x) (set (car x) (cdr x))))
 	       prio-list)
 	 (set-coding-priority (mapcar #'car prio-list))
+	 ;; Changing the binding of a coding category requires this call.
 	 (update-coding-systems-internal)
 	 (detect-coding-region ,from ,to))
      ;; We must restore the internal database.