diff --git a/guix/build/profiles.scm b/guix/build/profiles.scm
index 1dc7976879..67ee9b665a 100644
--- a/guix/build/profiles.scm
+++ b/guix/build/profiles.scm
@@ -1,5 +1,5 @@
 ;;; GNU Guix --- Functional package management for GNU
-;;; Copyright © 2015, 2017, 2018, 2019 Ludovic Courtès <ludo@gnu.org>
+;;; Copyright © 2015, 2017, 2018, 2019, 2020 Ludovic Courtès <ludo@gnu.org>
 ;;;
 ;;; This file is part of GNU Guix.
 ;;;
@@ -167,6 +167,10 @@ SEARCH-PATHS."
   ;; Store meta-data.
   (call-with-output-file manifest-file
     (lambda (p)
+      (display "\
+;; This file was automatically generated and is for internal use only.
+;; It cannot be passed to the '--manifest' option.\n\n"
+               p)
       (pretty-print manifest p)))
 
   ;; Make sure we can write to 'OUTPUT/etc'.  'union-build' above could have