diff --git a/lisp/emacs-lisp/bytecomp.el b/lisp/emacs-lisp/bytecomp.el
index 65ccb60726f..d093d95a775 100644
--- a/lisp/emacs-lisp/bytecomp.el
+++ b/lisp/emacs-lisp/bytecomp.el
@@ -1923,7 +1923,9 @@ also be compiled."
     (let ((directories (list default-directory))
 	  (default-directory default-directory)
           (ignore-files-regexp
-           (mapconcat #'identity byte-compile-ignore-files "\\|"))
+           (if byte-compile-ignore-files
+               (mapconcat #'identity byte-compile-ignore-files "\\|")
+             regexp-unmatchable))
 	  (skip-count 0)
 	  (fail-count 0)
 	  (file-count 0)