diff --git a/src/Symfony/Component/Routing/Generator/UrlGenerator.php b/src/Symfony/Component/Routing/Generator/UrlGenerator.php
index 095fcf127c..3b3548e26e 100644
--- a/src/Symfony/Component/Routing/Generator/UrlGenerator.php
+++ b/src/Symfony/Component/Routing/Generator/UrlGenerator.php
@@ -108,7 +108,7 @@ class UrlGenerator implements UrlGeneratorInterface
                         throw new \InvalidArgumentException(sprintf('Parameter "%s" for route "%s" must match "%s" ("%s" given).', $token[3], $name, $requirements[$token[3]], $tparams[$token[3]]));
                     }
 
-                    if (isset($tparams[$token[3]])) {
+                    if ($tparams[$token[3]] || !$optional) {
                         // %2F is not valid in a URL, so we don't encode it (which is fine as the requirements explicitly allowed it)
                         $url = $token[1].str_replace('%2F', '/', urlencode($tparams[$token[3]])).$url;
                     }
diff --git a/tests/Symfony/Tests/Component/Routing/Generator/UrlGeneratorTest.php b/tests/Symfony/Tests/Component/Routing/Generator/UrlGeneratorTest.php
index 03d9f997da..6434459362 100644
--- a/tests/Symfony/Tests/Component/Routing/Generator/UrlGeneratorTest.php
+++ b/tests/Symfony/Tests/Component/Routing/Generator/UrlGeneratorTest.php
@@ -74,6 +74,14 @@ class UrlGeneratorTest extends \PHPUnit_Framework_TestCase
         $this->assertEquals('/app.php/testing', $url);
     }
 
+    public function testRelativeUrlWithNullParameterButNotOptional()
+    {
+        $routes = $this->getRoutes('test', new Route('/testing/{foo}/bar', array('foo' => null)));
+        $url = $this->getGenerator($routes)->generate('test', array(), false);
+
+        $this->assertEquals('/app.php/testing//bar', $url);
+    }
+
     public function testRelativeUrlWithExtraParameters()
     {
         $routes = $this->getRoutes('test', new Route('/testing'));