diff --git a/src/Symfony/Bridge/Monolog/Handler/FirePHPHandler.php b/src/Symfony/Bridge/Monolog/Handler/FirePHPHandler.php
index b5906b18c2be3..27765fa848a8f 100644
--- a/src/Symfony/Bridge/Monolog/Handler/FirePHPHandler.php
+++ b/src/Symfony/Bridge/Monolog/Handler/FirePHPHandler.php
@@ -41,7 +41,7 @@ public function onKernelResponse(ResponseEvent $event)
         }
 
         $request = $event->getRequest();
-        if (!preg_match('{\bFirePHP/\d+\.\d+\b}', $request->headers->get('User-Agent'))
+        if (!preg_match('{\bFirePHP/\d+\.\d+\b}', $request->headers->get('User-Agent') ?? '')
             && !$request->headers->has('X-FirePHP-Version')) {
             self::$sendHeaders = false;
             $this->headers = [];