diff --git a/src/Servers/Reverb/Http/Response.php b/src/Servers/Reverb/Http/Response.php
index dd2c8db4..85858cb4 100644
--- a/src/Servers/Reverb/Http/Response.php
+++ b/src/Servers/Reverb/Http/Response.php
@@ -13,6 +13,6 @@ public function __construct(mixed $data = null, int $status = 200, array $header
     {
         parent::__construct($data, $status, $headers, $json);
 
-        $this->headers->set('Content-Length', strlen($this->content));
+        $this->headers->set('Content-Length', (string) strlen($this->content));
     }
 }