diff --git a/HttpKernel.php b/HttpKernel.php
index 0199b3d0d6..e6284670ae 100644
--- a/HttpKernel.php
+++ b/HttpKernel.php
@@ -61,7 +61,7 @@ public function __construct(EventDispatcherInterface $dispatcher, ControllerReso
      */
     public function handle(Request $request, int $type = HttpKernelInterface::MASTER_REQUEST, bool $catch = true)
     {
-        $request->headers->set('X-Php-Ob-Level', ob_get_level());
+        $request->headers->set('X-Php-Ob-Level', strval(ob_get_level()));
 
         try {
             return $this->handleRaw($request, $type);