diff --git a/DependencyInjection/KnpMarkdownExtension.php b/DependencyInjection/KnpMarkdownExtension.php
index 28918be..c3d32ef 100644
--- a/DependencyInjection/KnpMarkdownExtension.php
+++ b/DependencyInjection/KnpMarkdownExtension.php
@@ -3,6 +3,7 @@
 namespace Knp\Bundle\MarkdownBundle\DependencyInjection;
 
 use Symfony\Component\Config\Definition\Processor;
+use Symfony\Component\Config\Definition\Exception\InvalidConfigurationException;
 use Symfony\Component\HttpKernel\DependencyInjection\Extension;
 use Symfony\Component\DependencyInjection\Loader\XmlFileLoader;
 use Symfony\Component\Config\FileLocator;
@@ -30,5 +31,9 @@ class KnpMarkdownExtension extends Extension
         $container->setParameter('markdown.sundown.extensions', $config['sundown']['extensions']);
         $container->setParameter('markdown.sundown.render_flags', $config['sundown']['render_flags']);
         $container->setAlias('markdown.parser', $config['parser']['service']);
+
+        if ($config['parser']['service'] == 'markdown.parser.sundown' && !class_exists('Sundown\Markdown')) {
+                throw new InvalidConfigurationException('<strong>Sundown</strong> extension not installed or configured.');
+        }
     }
 }