diff --git a/lisp/custom.el b/lisp/custom.el
index 076790b661..e55c9db1ac 100644
--- a/lisp/custom.el
+++ b/lisp/custom.el
@@ -1288,10 +1288,13 @@ custom-available-themes
 loaded, and no effort is made to check that the files contain
 valid Custom themes.  For a list of loaded themes, check the
 variable `custom-known-themes'."
+  (eval-when-compile (require 'cl-lib))
   (let ((suffix "-theme\\.el\\'")
         themes)
     (dolist (dir (custom-theme--load-path))
-      ;; `custom-theme--load-path' promises DIR exists and is a directory.
+      (cl-assert
+       (file-directory-p dir) t
+       "Non-existent directory in `custom-theme-load-path' expansion: %s")
       (dolist (file (directory-files dir nil suffix))
         (let ((theme (intern (substring file 0 (string-match-p suffix file)))))
           (and (custom-theme-name-valid-p theme)