diff --git a/src/Bundle/Resources/contao/config/config.php b/src/Bundle/Resources/contao/config/config.php
index 9f4431c..3603cf1 100644
--- a/src/Bundle/Resources/contao/config/config.php
+++ b/src/Bundle/Resources/contao/config/config.php
@@ -44,7 +44,7 @@ array_insert(
     ]
 );
 
-if (TL_MODE === 'BE') {
+if (defined('TL_MODE') && TL_MODE === 'BE') {
     $GLOBALS['TL_CSS'][] = 'bundles/netzmachtcontaoleaflet/css/backend_global.css';
 }