diff --git a/flag.inc b/flag.inc
index b1a54a1..a24c9c6 100644
--- a/flag.inc
+++ b/flag.inc
@@ -812,6 +812,9 @@ class flag_flag {
    * @private
    */
   function _flag($content_id, $uid, $sid) {
+    if ($uid == 0) {
+      $sid = REQUEST_TIME;
+    }
     $fcid = db_insert('flag_content')
       ->fields(array(
         'fid' => $this->fid,