diff --git a/core/lib/Drupal/Core/StackMiddleware/ReverseProxyMiddleware.php b/core/lib/Drupal/Core/StackMiddleware/ReverseProxyMiddleware.php
index c1e62b340e..9894a729d5 100644
--- a/core/lib/Drupal/Core/StackMiddleware/ReverseProxyMiddleware.php
+++ b/core/lib/Drupal/Core/StackMiddleware/ReverseProxyMiddleware.php
@@ -62,7 +62,7 @@ public function handle(Request $request, $type = self::MASTER_REQUEST, $catch =
   public static function setSettingsOnRequest(Request $request, Settings $settings) {
     // Initialize proxy settings.
     if ($settings->get('reverse_proxy', FALSE)) {
-      if (TRUE === func_get_arg(3)) {
+      if (3 === func_num_args() && TRUE === func_get_arg(3)) {
         @trigger_error('The "' . __NAMESPACE__ . '\\' . __CLASS__ . '" class has been deprecated in Drupal 8.6.1 and will be removed before Drupal 9.0.0. Use the new configuration file "sites/reverse_proxy_settings.php" instead. See https://www.drupal.org/project/drupal/issues/2998728.', E_USER_DEPRECATED);
       }