diff --git a/lib/Twig/Environment.php b/lib/Twig/Environment.php
index 55aef05b8..1596d0add 100644
--- a/lib/Twig/Environment.php
+++ b/lib/Twig/Environment.php
@@ -462,7 +462,7 @@ class Twig_Environment
         if (0 === $this->lastModifiedExtension) {
             foreach ($this->extensions as $extension) {
                 $r = new ReflectionObject($extension);
-                if (($extensionTime = filemtime($r->getFileName())) > $this->lastModifiedExtension) {
+                if (file_exists($r->getFileName()) && ($extensionTime = filemtime($r->getFileName())) > $this->lastModifiedExtension) {
                     $this->lastModifiedExtension = $extensionTime;
                 }
             }
diff --git a/test/Twig/Tests/EnvironmentTest.php b/test/Twig/Tests/EnvironmentTest.php
index 57b91ae50..a43e16e2b 100644
--- a/test/Twig/Tests/EnvironmentTest.php
+++ b/test/Twig/Tests/EnvironmentTest.php
@@ -292,6 +292,22 @@ class Twig_Tests_EnvironmentTest extends PHPUnit_Framework_TestCase
         $this->assertCount(2, $twig->getNodeVisitors());
     }
 
+    public function testAddMockExtension()
+    {
+        $extension = $this->getMock('Twig_ExtensionInterface');
+        $extension->expects($this->once())
+            ->method('getName')
+            ->will($this->returnValue('mock'));
+
+        $loader = new Twig_Loader_Array(array('page' => 'hey'));
+
+        $twig = new Twig_Environment($loader);
+        $twig->addExtension($extension);
+
+        $this->assertInstanceOf('Twig_ExtensionInterface', $twig->getExtension('mock'));
+        $this->assertTrue($twig->isTemplateFresh('page', time()));
+    }
+
     protected function getMockLoader($templateName, $templateContent)
     {
         $loader = $this->getMock('Twig_LoaderInterface');