diff --git a/src/Symfony/Bundle/FrameworkBundle/DependencyInjection/Configuration.php b/src/Symfony/Bundle/FrameworkBundle/DependencyInjection/Configuration.php
index 936b9cf00dd8e..909e1655570ae 100644
--- a/src/Symfony/Bundle/FrameworkBundle/DependencyInjection/Configuration.php
+++ b/src/Symfony/Bundle/FrameworkBundle/DependencyInjection/Configuration.php
@@ -554,6 +554,7 @@ private function addAssetsSection(ArrayNodeDefinition $rootNode)
                                 ->fixXmlConfig('base_url')
                                 ->children()
                                     ->scalarNode('version')
+                                        ->defaultNull()
                                         ->beforeNormalization()
                                         ->ifTrue(function ($v) { return '' === $v; })
                                         ->then(function ($v) { return; })