diff --git a/Makefile b/Makefile
index 341211af..74d63193 100644
--- a/Makefile
+++ b/Makefile
@@ -12,7 +12,7 @@ vendor/composer/installed.json: composer.json composer.lock
 
 .PHONY: phpunit
 phpunit:
-	@vendor/bin/phpunit
+	@php -dzend.assertions=1 vendor/bin/phpunit
 
 .PHONY: infection
 infection:
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();
     }