diff --git a/modules/user/user.test b/modules/user/user.test
index 5aa03629642f..a57983d18027 100644
--- a/modules/user/user.test
+++ b/modules/user/user.test
@@ -1016,7 +1016,7 @@ class UserBlocksUnitTests extends DrupalWebTestCase {
   private function insertSession(array $fields = array()) {
     $fields += array(
       'uid' => 0,
-      'sid' => md5(microtime()),
+      'sid' => md5(uniqid(mt_rand(), TRUE)),
       'timestamp' => REQUEST_TIME,
     );
     db_insert('sessions')