diff --git a/lisp/custom.el b/lisp/custom.el
index b3311a1783..d21e398646 100644
--- a/lisp/custom.el
+++ b/lisp/custom.el
@@ -1292,6 +1292,7 @@ custom-available-themes
   (let ((suffix "-theme\\.el\\'")
         themes)
     (dolist (dir (custom-theme--load-path))
+      ;; `custom-theme--load-path' promises DIR exists.
       (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)
@@ -1300,6 +1301,8 @@ custom-available-themes
     (nreverse themes)))
 
 (defun custom-theme--load-path ()
+  "Expand `custom-theme-load-path' into list of directories.
+Only existing directories are included in the path returned."
   (let (lpath)
     (dolist (f custom-theme-load-path)
       (cond ((eq f 'custom-theme-directory)