diff --git a/core/lib/Drupal/Core/StackMiddleware/NegotiationMiddleware.php b/core/lib/Drupal/Core/StackMiddleware/NegotiationMiddleware.php
index 9f6f90d..75df1cb 100644
--- a/core/lib/Drupal/Core/StackMiddleware/NegotiationMiddleware.php
+++ b/core/lib/Drupal/Core/StackMiddleware/NegotiationMiddleware.php
@@ -38,7 +38,7 @@ class NegotiationMiddleware implements HttpKernelInterface {
    *
    * @var array
    */
-  protected $formats;
+  protected $formats = [];
 
   /**
    * Constructs a new NegotiationMiddleware.