diff --git a/plugins/system/sef/src/Extension/Sef.php b/plugins/system/sef/src/Extension/Sef.php
index 0480cd978a647..951e1d51cf227 100644
--- a/plugins/system/sef/src/Extension/Sef.php
+++ b/plugins/system/sef/src/Extension/Sef.php
@@ -118,7 +118,7 @@ public function onAfterRoute(AfterRouteEvent $event)
         if ($router->isTainted()) {
             $parsedVars = $router->getVars();
 
-            if ($app->getLanguageFilter()) {
+            if ($app->getLanguageFilter() && isset($parsedVars['language'])) {
                 $parsedVars['lang'] = $parsedVars['language'];
                 unset($parsedVars['language']);
             }