diff --git a/lisp/ob-tangle.el b/lisp/ob-tangle.el
index 7484ffe76..3a3874556 100644
--- a/lisp/ob-tangle.el
+++ b/lisp/ob-tangle.el
@@ -327,8 +327,9 @@ Did you give the decimal value %1$d by mistake?" mode)))
    ((string-match-p "^o0?[0-7][0-7][0-7]$" mode)
     (string-to-number (replace-regexp-in-string "^o" "" mode) 8))
    ((string-match-p "^[ugoa]*\\(?:[+-=][rwxXstugo]*\\)+\\(,[ugoa]*\\(?:[+-=][rwxXstugo]*\\)+\\)*$" mode)
+    ;; Match regexp taken from `file-modes-symbolic-to-number'.
     (file-modes-symbolic-to-number mode org-babel-tangle-default-mode))
-   ((string-match-p "^[rwx-]\\{9\\}$" mode)
+   ((string-match-p "^\\(?:[r-][w-][x-]\\)\\{3\\}$" mode)
     (file-modes-symbolic-to-number (concat  "u=" (substring mode 0 3)
                                             ",g=" (substring mode 3 6)
                                             ",a=" (substring mode 6 9))