diff --git a/core/lib/Drupal/Core/EventSubscriber/FinishResponseSubscriber.php b/core/lib/Drupal/Core/EventSubscriber/FinishResponseSubscriber.php
index 94e64a4..57210c2 100644
--- a/core/lib/Drupal/Core/EventSubscriber/FinishResponseSubscriber.php
+++ b/core/lib/Drupal/Core/EventSubscriber/FinishResponseSubscriber.php
@@ -115,17 +115,25 @@ public function onRespond(FilterResponseEvent $event) {
     $response->headers->set('X-Content-Type-Options', 'nosniff', FALSE);
     $response->headers->set('X-Frame-Options', 'SAMEORIGIN', FALSE);
 
+    // If the current response isn't an implementation of the
+    // CacheableResponseInterface, we're trusting that a Response is either
+    // explicitly not cacheable or that caching headers are already set from
+    // another place.
+    if (!($response instanceof CacheableResponseInterface)) {
+      return;
+    }
+
     // Expose the cache contexts and cache tags associated with this page in a
     // X-Drupal-Cache-Contexts and X-Drupal-Cache-Tags header respectively, when
     // either a reverse proxy is being used (so the reverse proxy or CDN can be
     // invalidated when appropriate) or when developing/debugging.
-    if ((Settings::get('reverse_proxy', FALSE) || Settings::get('send_cacheability_headers', FALSE)) && $response instanceof CacheableResponseInterface) {
+    if (Settings::get('reverse_proxy', FALSE) || Settings::get('send_cacheability_headers', FALSE)) {
       $response_cacheability = $response->getCacheableMetadata();
       $response->headers->set('X-Drupal-Cache-Tags', implode(' ', $response_cacheability->getCacheTags()));
       $response->headers->set('X-Drupal-Cache-Contexts', implode(' ', $this->cacheContextsManager->optimizeTokens($response_cacheability->getCacheContexts())));
     }
 
-    $is_cacheable = ($response instanceof CacheableResponseInterface && $this->requestPolicy->check($request) === RequestPolicyInterface::ALLOW) && ($this->responsePolicy->check($response, $request) !== ResponsePolicyInterface::DENY);
+    $is_cacheable = ($this->requestPolicy->check($request) === RequestPolicyInterface::ALLOW) && ($this->responsePolicy->check($response, $request) !== ResponsePolicyInterface::DENY);
 
     // Add headers necessary to specify whether the response should be cached by
     // proxies and/or the browser.