diff --git a/lib/tablelib.php b/lib/tablelib.php
index 712c8b505cd0a..06612d7991b7c 100644
--- a/lib/tablelib.php
+++ b/lib/tablelib.php
@@ -220,10 +220,10 @@ function __construct($uniqueid) {
      * for you (even if the param is '', which means no download this time.
      * Also you can call this method with no params to get the current set
      * download type.
-     * @param string $download dataformat type. One of csv, xhtml, ods, etc
+     * @param string|null $download type of dataformat for export.
      * @param string $filename filename for downloads without file extension.
      * @param string $sheettitle title for downloaded data.
-     * @return string download dataformat type. One of csv, xhtml, ods, etc
+     * @return string download dataformat type.
      */
     function is_downloading($download = null, $filename='', $sheettitle='') {
         if ($download!==null) {