diff --git a/src/Symfony/Bundle/FrameworkBundle/Routing/Router.php b/src/Symfony/Bundle/FrameworkBundle/Routing/Router.php
index 3f9b5cf75e..c69ec13066 100644
--- a/src/Symfony/Bundle/FrameworkBundle/Routing/Router.php
+++ b/src/Symfony/Bundle/FrameworkBundle/Routing/Router.php
@@ -114,6 +114,10 @@ class Router extends BaseRouter implements WarmableInterface
     {
         $container = $this->container;
 
+        if (null === $value) {
+            return null;
+        }
+
         $escapedValue = preg_replace_callback('/%%|%([^%\s]+)%/', function ($match) use ($container, $value) {
             // skip %%
             if (!isset($match[1])) {