core/lib/Drupal/Core/EventSubscriber/DefaultExceptionSubscriber.php  | 2 +-
 .../Functional/EntityResource/Comment/CommentResourceTestBase.php    | 5 +++--
 .../tests/src/Functional/EntityResource/EntityResourceTestBase.php   | 2 +-
 3 files changed, 5 insertions(+), 4 deletions(-)

diff --git a/core/lib/Drupal/Core/EventSubscriber/DefaultExceptionSubscriber.php b/core/lib/Drupal/Core/EventSubscriber/DefaultExceptionSubscriber.php
index 3ae147d..99451df 100644
--- a/core/lib/Drupal/Core/EventSubscriber/DefaultExceptionSubscriber.php
+++ b/core/lib/Drupal/Core/EventSubscriber/DefaultExceptionSubscriber.php
@@ -119,7 +119,7 @@ protected function onHtml(GetResponseForExceptionEvent $event) {
 
     $content = $this->t('The website encountered an unexpected error. Please try again later.');
     $content .= $message ? '</br></br>' . $message : '';
-    $response = new Response($content, 500);
+    $response = new Response($content, 500, ['Content-Type' => 'text/plain']);
 
     if ($exception instanceof HttpExceptionInterface) {
       $response->setStatusCode($exception->getStatusCode());
diff --git a/core/modules/rest/tests/src/Functional/EntityResource/Comment/CommentResourceTestBase.php b/core/modules/rest/tests/src/Functional/EntityResource/Comment/CommentResourceTestBase.php
index f6645f4..2b7c5a3 100644
--- a/core/modules/rest/tests/src/Functional/EntityResource/Comment/CommentResourceTestBase.php
+++ b/core/modules/rest/tests/src/Functional/EntityResource/Comment/CommentResourceTestBase.php
@@ -303,8 +303,9 @@ public function testPostDxWithoutCriticalBaseFields() {
     // DX: 422 when missing 'entity_type' field.
     $request_options[RequestOptions::BODY] = $this->serializer->encode(array_diff_key($this->getNormalizedPostEntity(), ['field_name' => TRUE]), static::$format);
     $response = $this->request('POST', $url, $request_options);
-    // @todo Uncomment, remove next line in https://www.drupal.org/node/2820364.
-    $this->assertResourceErrorResponse(500, FALSE, $response);
+    // @todo Uncomment, remove next 2 lines in https://www.drupal.org/node/2820364.
+    $this->assertSame(500, $response->getStatusCode());
+    $this->assertSame(['text/plain; charset=UTF-8'], $response->getHeader('Content-Type'));
     //$this->assertResourceErrorResponse(422, "Unprocessable Entity: validation failed.\nfield_name: This value should not be null.\n", $response);
   }
 
diff --git a/core/modules/rest/tests/src/Functional/EntityResource/EntityResourceTestBase.php b/core/modules/rest/tests/src/Functional/EntityResource/EntityResourceTestBase.php
index 0c10678..9f326bc 100644
--- a/core/modules/rest/tests/src/Functional/EntityResource/EntityResourceTestBase.php
+++ b/core/modules/rest/tests/src/Functional/EntityResource/EntityResourceTestBase.php
@@ -508,7 +508,7 @@ public function testGet() {
 
 
     // DX: 406 when requesting unsupported format but specifying Accept header:
-    // should still result in a response with the same Content-Type.
+    // should result in a text/plain response.
     $response = $this->request('GET', $url, $request_options);
     $this->assert406Response($response);
     $this->assertSame(['text/plain; charset=UTF-8'], $response->getHeader('Content-Type'));