diff --git a/src/ServiceContainer/SymfonyExtension.php b/src/ServiceContainer/SymfonyExtension.php
index 7eb8b60..0c08971 100644
--- a/src/ServiceContainer/SymfonyExtension.php
+++ b/src/ServiceContainer/SymfonyExtension.php
@@ -277,7 +277,7 @@ final class SymfonyExtension implements Extension
      *
      * @throws \DomainException
      */
-    private function requireKernelBootstrapFile(string $basePath, string $bootstrapPath): void
+    private function requireKernelBootstrapFile(string $basePath, ?string $bootstrapPath): void
     {
         if (null === $bootstrapPath) {
             return;