diff --git a/flag.module b/flag.module
index 5b50d3b..55fbbef 100644
--- a/flag.module
+++ b/flag.module
@@ -2058,7 +2058,9 @@ function flag_set_sid($uid = NULL, $create = TRUE) {
     $uid = $GLOBALS['user']->uid;
   }
 
-  if (!isset($sids[$uid])) {
+  // Create an SID of none has been set, yet. Also create one if previously
+  // none was required (-1) but now it is required ($create == TRUE).
+  if (!isset($sids[$uid]) || ($sids[$uid] == -1 && $create)) {
     if (module_exists('session_api') && session_api_available() && $uid == 0) {
       $sids[$uid] = session_api_get_sid($create);
     }
@@ -2067,7 +2069,9 @@ function flag_set_sid($uid = NULL, $create = TRUE) {
     }
   }
 
-  return $sids[$uid];
+  // Keep the -1 case internal and let the outside world only distinguish two
+  // cases: (1) there is an SID; (2) there is no SID (-> 0).
+  return $sids[$uid] != -1 ? $sids[$uid] : 0;
 }
 
 /**