diff --git a/core/lib/Drupal/Core/EventSubscriber/ActiveLinkResponseFilter.php b/core/lib/Drupal/Core/EventSubscriber/ActiveLinkResponseFilter.php
index 4e530cf31bfe0ac86dcc059ca8c8c836ab4c8491..bb3031e6e90b700e1011ddb681b063ab0047f400 100644
--- a/core/lib/Drupal/Core/EventSubscriber/ActiveLinkResponseFilter.php
+++ b/core/lib/Drupal/Core/EventSubscriber/ActiveLinkResponseFilter.php
@@ -79,7 +79,7 @@ public function onResponse(ResponseEvent $event) {
     $response = $event->getResponse();
 
     // Only care about HTML responses.
-    if (stripos($response->headers->get('Content-Type'), 'text/html') === FALSE) {
+    if (stripos($response->headers->get('Content-Type', ''), 'text/html') === FALSE) {
       return;
     }
 
diff --git a/core/lib/Drupal/Core/EventSubscriber/AjaxResponseSubscriber.php b/core/lib/Drupal/Core/EventSubscriber/AjaxResponseSubscriber.php
index 10d1426485767aea89c990aae741522e65e973b6..5f730dcf4d4f002efec16cd9f366c1fbd183f9df 100644
--- a/core/lib/Drupal/Core/EventSubscriber/AjaxResponseSubscriber.php
+++ b/core/lib/Drupal/Core/EventSubscriber/AjaxResponseSubscriber.php
@@ -83,7 +83,7 @@ public function onResponse(ResponseEvent $event) {
       // @see https://www.drupal.org/node/1009382
       // @see https://www.drupal.org/node/2339491
       // @see Drupal.ajax.prototype.beforeSend()
-      $accept = $event->getRequest()->headers->get('accept');
+      $accept = $event->getRequest()->headers->get('accept', '');
 
       if (strpos($accept, 'text/html') !== FALSE) {
         $response->headers->set('Content-Type', 'text/html; charset=utf-8');
diff --git a/core/lib/Drupal/Core/EventSubscriber/RssResponseRelativeUrlFilter.php b/core/lib/Drupal/Core/EventSubscriber/RssResponseRelativeUrlFilter.php
index d9f8eaad611deb7a7997214d875d00afe81f41f9..5da62bd82382938dad44f8e09d15556da6576bbf 100644
--- a/core/lib/Drupal/Core/EventSubscriber/RssResponseRelativeUrlFilter.php
+++ b/core/lib/Drupal/Core/EventSubscriber/RssResponseRelativeUrlFilter.php
@@ -21,7 +21,7 @@ class RssResponseRelativeUrlFilter implements EventSubscriberInterface {
    */
   public function onResponse(ResponseEvent $event) {
     // Only care about RSS responses.
-    if (stripos($event->getResponse()->headers->get('Content-Type'), 'application/rss+xml') === FALSE) {
+    if (stripos($event->getResponse()->headers->get('Content-Type', ''), 'application/rss+xml') === FALSE) {
       return;
     }
 
diff --git a/core/modules/jsonapi/src/EventSubscriber/ResourceResponseValidator.php b/core/modules/jsonapi/src/EventSubscriber/ResourceResponseValidator.php
index 16250682e95671217cc86593c11d6ed35b31b684..1ba0cd0196c9b3c1a60073cdd945362c40737066 100644
--- a/core/modules/jsonapi/src/EventSubscriber/ResourceResponseValidator.php
+++ b/core/modules/jsonapi/src/EventSubscriber/ResourceResponseValidator.php
@@ -101,7 +101,7 @@ public function setValidator(Validator $validator = NULL) {
    */
   public function onResponse(ResponseEvent $event) {
     $response = $event->getResponse();
-    if (strpos($response->headers->get('Content-Type'), 'application/vnd.api+json') === FALSE) {
+    if (strpos($response->headers->get('Content-Type', ''), 'application/vnd.api+json') === FALSE) {
       return;
     }