diff --git a/lisp/custom.el b/lisp/custom.el
index 076790b661..f08a8e9c3e 100644
--- a/lisp/custom.el
+++ b/lisp/custom.el
@@ -1291,7 +1291,9 @@ custom-available-themes
   (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)