diff --git a/lisp/dired.el b/lisp/dired.el
index 5b5b6f073df..e023da27be3 100644
--- a/lisp/dired.el
+++ b/lisp/dired.el
@@ -2806,7 +2806,9 @@ It runs the hook `dired-initial-position-hook'."
 (defun dired-current-directory (&optional localp)
   "Return the name of the subdirectory to which this line belongs.
 This returns a string with trailing slash, like `default-directory'.
-Optional argument means return a file name relative to `default-directory'."
+Optional argument means return a file name relative to `default-directory',
+in which case the value could be an empty string if `default-directory'
+is the directory where the file on this line resides."
   (let ((here (point))
 	(alist (or dired-subdir-alist
 		   ;; probably because called in a non-dired buffer