diff --git a/src/Generator.php b/src/Generator.php
index 36b6f76c..be728d4c 100644
--- a/src/Generator.php
+++ b/src/Generator.php
@@ -44,7 +44,7 @@ private function loadContainer(ContainerConfiguration $config, ConfigCache $dump
     {
         require_once $dump->getPath();
         $className = $config->getClassName();
-        assert(is_a($className, ContainerInterface::class));
+        assert(is_a($className, ContainerInterface::class, true));
 
         return new $className();
     }