diff --git a/src/Symfony/Component/HttpFoundation/Session/Storage/Handler/MongoDbSessionHandler.php b/src/Symfony/Component/HttpFoundation/Session/Storage/Handler/MongoDbSessionHandler.php
index 6006773c9d..e8293e4dde 100644
--- a/src/Symfony/Component/HttpFoundation/Session/Storage/Handler/MongoDbSessionHandler.php
+++ b/src/Symfony/Component/HttpFoundation/Session/Storage/Handler/MongoDbSessionHandler.php
@@ -104,7 +104,7 @@ class MongoDbSessionHandler implements \SessionHandlerInterface
     {
         $data = array(
             $this->options['id_field']   => $sessionId,
-            $this->options['data_field'] => new \MongoBinData($data),
+            $this->options['data_field'] => new \MongoBinData($data, \MongoBinData::BYTE_ARRAY),
             $this->options['time_field'] => new \MongoTimestamp()
         );