.../Drupal/Core/EventSubscriber/DefaultExceptionSubscriber.php | 10 ----------
 .../EntityResource/Comment/CommentResourceTestBase.php         | 10 ++++------
 .../src/Functional/EntityResource/EntityResourceTestBase.php   |  8 ++++----
 3 files changed, 8 insertions(+), 20 deletions(-)

diff --git a/core/lib/Drupal/Core/EventSubscriber/DefaultExceptionSubscriber.php b/core/lib/Drupal/Core/EventSubscriber/DefaultExceptionSubscriber.php
index 4737e80..836b667 100644
--- a/core/lib/Drupal/Core/EventSubscriber/DefaultExceptionSubscriber.php
+++ b/core/lib/Drupal/Core/EventSubscriber/DefaultExceptionSubscriber.php
@@ -218,16 +218,6 @@ protected function getFormat(Request $request) {
       $format = 'json';
     }
 
-    // Make an educated guess that any Accept header type that includes "json"
-    // can probably handle a generic JSON response for errors. As above, for
-    // any format this doesn't catch or that wants custom handling should
-    // register its own exception listener.
-    foreach ($request->getAcceptableContentTypes() as $mime) {
-      if (strpos($mime, 'html') === FALSE && strpos($mime, 'json') !== FALSE) {
-        $format = 'json';
-      }
-    }
-
     return $format;
   }
 
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 7e38b33..f6645f4 100644
--- a/core/modules/rest/tests/src/Functional/EntityResource/Comment/CommentResourceTestBase.php
+++ b/core/modules/rest/tests/src/Functional/EntityResource/Comment/CommentResourceTestBase.php
@@ -280,8 +280,8 @@ public function testPostDxWithoutCriticalBaseFields() {
     $response = $this->request('POST', $url, $request_options);
     // @todo Uncomment, remove next 3 lines in https://www.drupal.org/node/2820364.
     $this->assertSame(500, $response->getStatusCode());
-    $this->assertSame(['application/json'], $response->getHeader('Content-Type'));
-    $this->assertSame('{"message":"A fatal error occurred: Internal Server Error"}', (string) $response->getBody());
+    $this->assertSame(['text/plain; charset=UTF-8'], $response->getHeader('Content-Type'));
+    $this->assertSame('Internal Server Error', (string) $response->getBody());
     //$this->assertResourceErrorResponse(422, "Unprocessable Entity: validation failed.\nentity_type: This value should not be null.\n", $response);
 
     // DX: 422 when missing 'entity_id' field.
@@ -303,10 +303,8 @@ 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 3 lines in https://www.drupal.org/node/2820364.
-    $this->assertSame(500, $response->getStatusCode());
-    $this->assertSame(['application/json'], $response->getHeader('Content-Type'));
-    $this->assertSame('{"message":"A fatal error occurred: Field  is unknown."}', (string) $response->getBody());
+    // @todo Uncomment, remove next line in https://www.drupal.org/node/2820364.
+    $this->assertResourceErrorResponse(500, FALSE, $response);
     //$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 73dfc62..0c10678 100644
--- a/core/modules/rest/tests/src/Functional/EntityResource/EntityResourceTestBase.php
+++ b/core/modules/rest/tests/src/Functional/EntityResource/EntityResourceTestBase.php
@@ -501,17 +501,17 @@ public function testGet() {
     // DX: 406 when requesting unsupported format.
     $response = $this->request('GET', $url, $request_options);
     $this->assert406Response($response);
-    $this->assertNotSame([static::$mimeType], $response->getHeader('Content-Type'));
+    $this->assertSame(['text/plain; charset=UTF-8'], $response->getHeader('Content-Type'));
 
 
     $request_options[RequestOptions::HEADERS]['Accept'] = static::$mimeType;
 
 
-    // DX: 406 when requesting unsupported format but specifying Accept header.
-    // @todo Update in https://www.drupal.org/node/2825347.
+    // DX: 406 when requesting unsupported format but specifying Accept header:
+    // should still result in a response with the same Content-Type.
     $response = $this->request('GET', $url, $request_options);
     $this->assert406Response($response);
-    $this->assertSame(['application/json'], $response->getHeader('Content-Type'));
+    $this->assertSame(['text/plain; charset=UTF-8'], $response->getHeader('Content-Type'));
 
 
     $url = Url::fromRoute('rest.entity.' . static::$entityTypeId . '.GET.' . static::$format);