Request::HEADER_X_FORWARDED_ALL is an integer

it is int(30)

app/Http/Middleware/TrustProxies.php

@@ -17,7 +17,7 @@ class TrustProxies extends Middleware
     /**
      * The headers that should be used to detect proxies.
      *
-     * @var string
+     * @var integer
      */
     protected $headers = Request::HEADER_X_FORWARDED_ALL;
 }