diff --git a/includes/session.inc b/includes/session.inc
index fd04de8..faa079c 100644
--- a/includes/session.inc
+++ b/includes/session.inc
@@ -172,7 +172,7 @@ function _drupal_session_write($sid, $value) {
 
     // For performance reasons, do not update the sessions table, unless
     // $_SESSION has changed or more than 180 has passed since the last update.
-    if ($is_changed || REQUEST_TIME - $user->timestamp > variable_get('session_write_interval', 180)) {
+    if ($is_changed || !isset($user->timestamp) || REQUEST_TIME - $user->timestamp > variable_get('session_write_interval', 180)) {
       // Either ssid or sid or both will be added from $key below.
       $fields = array(
         'uid' => $user->uid,