diff --git a/jsonapi.services.yml b/jsonapi.services.yml
index 4b65c59..09e21d3 100644
--- a/jsonapi.services.yml
+++ b/jsonapi.services.yml
@@ -102,9 +102,9 @@ services:
   jsonapi.http_middleware.format_setter:
     class: Drupal\jsonapi\StackMiddleware\FormatSetter
     tags:
-    # Set priority to 399 so it happens right after the format in the ?_format
-    # has been set in the request. That is set to 400.
-      - { name: http_middleware, priority: 399, responder: true }
+    # Set priority to 201 so it happens right before the page cache middleware
+    # has the opportunity to respond. http_middleware.page_cache is 200.
+      - { name: http_middleware, priority: 201, responder: true }
 
   logger.channel.jsonapi:
     parent: logger.channel_base
diff --git a/src/EventSubscriber/DefaultExceptionSubscriber.php b/src/EventSubscriber/DefaultExceptionSubscriber.php
index ef57d37..cb85137 100644
--- a/src/EventSubscriber/DefaultExceptionSubscriber.php
+++ b/src/EventSubscriber/DefaultExceptionSubscriber.php
@@ -3,6 +3,7 @@
 namespace Drupal\jsonapi\EventSubscriber;
 
 use Drupal\serialization\EventSubscriber\DefaultExceptionSubscriber as SerializationDefaultExceptionSubscriber;
+use Symfony\Cmf\Component\Routing\RouteObjectInterface;
 use Symfony\Component\HttpFoundation\Request;
 use Symfony\Component\HttpFoundation\Response;
 use Symfony\Component\HttpKernel\Event\GetResponseForExceptionEvent;
@@ -69,10 +70,11 @@ class DefaultExceptionSubscriber extends SerializationDefaultExceptionSubscriber
    *   TRUE if it needs to be formated using JSON API. FALSE otherwise.
    */
   protected function isJsonApiFormatted(Request $request) {
+    $route = $request->attributes->get(RouteObjectInterface::ROUTE_OBJECT);
     $format = $request->getRequestFormat();
     // The JSON API format is supported if the format is explicitly set or the
     // request is for a known JSON API route.
-    return $format == 'api_json' || $request->attributes->get('_is_jsonapi');
+    return $format == 'api_json' || ($route && $route->getOption('_is_jsonapi'));
   }
 
 }