.../DefaultExceptionSubscriberTest.php             | 80 +++++++++++++++++++++-
 1 file changed, 78 insertions(+), 2 deletions(-)

diff --git a/core/modules/serialization/tests/src/Unit/EventSubscriber/DefaultExceptionSubscriberTest.php b/core/modules/serialization/tests/src/Unit/EventSubscriber/DefaultExceptionSubscriberTest.php
index e20f6f7..bead596 100644
--- a/core/modules/serialization/tests/src/Unit/EventSubscriber/DefaultExceptionSubscriberTest.php
+++ b/core/modules/serialization/tests/src/Unit/EventSubscriber/DefaultExceptionSubscriberTest.php
@@ -1,6 +1,6 @@
 <?php
 
-namespace Drupal\Tests\serialization\Unit\EventSubscriber;
+namespace Drupal\Tests\serialization\Unit\EventSubscriber {
 
 use Drupal\serialization\Encoder\JsonEncoder;
 use Drupal\serialization\EventSubscriber\DefaultExceptionSubscriber;
@@ -8,6 +8,7 @@
 use Symfony\Component\HttpFoundation\Request;
 use Symfony\Component\HttpFoundation\Response;
 use Symfony\Component\HttpKernel\Event\GetResponseForExceptionEvent;
+use Symfony\Component\HttpKernel\Exception\HttpException;
 use Symfony\Component\HttpKernel\Exception\MethodNotAllowedHttpException;
 use Symfony\Component\HttpKernel\HttpKernelInterface;
 use Symfony\Component\Serializer\Serializer;
@@ -28,7 +29,7 @@ public function testOn4xx() {
 
     $e = new MethodNotAllowedHttpException(['POST', 'PUT'], 'test message');
     $event = new GetResponseForExceptionEvent($kernel->reveal(), $request, 'GET', $e);
-    $subscriber = new DefaultExceptionSubscriber(new Serializer([], [new JsonEncoder()]), []);
+    $subscriber = new DefaultExceptionSubscriber(new Serializer([], [new JsonEncoder()]), ['json']);
     $subscriber->on4xx($event);
     $response = $event->getResponse();
 
@@ -39,4 +40,79 @@ public function testOn4xx() {
     $this->assertEquals('application/json', $response->headers->get('Content-Type'));
   }
 
+  /**
+   * @covers ::on500
+   */
+  public function testOn500() {
+    $kernel = $this->prophesize(HttpKernelInterface::class);
+    $request = Request::create('/test');
+    $request->setRequestFormat('json');
+
+    $e = new HttpException(500, 'Internal Server Error');
+    $event = new GetResponseForExceptionEvent($kernel->reveal(), $request, 'GET', $e);
+    $subscriber = new DefaultExceptionSubscriber(new Serializer([], [new JsonEncoder()]), ['json']);
+    $subscriber->on500($event);
+    $response = $event->getResponse();
+
+    $this->assertInstanceOf(Response::class, $response);
+    $this->assertEquals('{"message":"Internal Server Error"}', $response->getContent());
+    $this->assertEquals(500, $response->getStatusCode());
+    $this->assertEquals('application/json', $response->headers->get('Content-Type'));
+  }
+
+  /**
+   * @covers ::on500
+   */
+  public function testOnFatalErrorDefault() {
+    $kernel = $this->prophesize(HttpKernelInterface::class);
+    $request = Request::create('/test');
+    $request->setRequestFormat('json');
+
+    $e = new \InvalidArgumentException('Some explanation here.');
+    $event = new GetResponseForExceptionEvent($kernel->reveal(), $request, 'GET', $e);
+    $subscriber = new DefaultExceptionSubscriber(new Serializer([], [new JsonEncoder()]), ['json']);
+    $subscriber->onException($event);
+    $response = $event->getResponse();
+
+    $this->assertInstanceOf(Response::class, $response);
+    $this->assertEquals('{"message":"A fatal error occurred."}', $response->getContent());
+    $this->assertEquals(500, $response->getStatusCode());
+    $this->assertEquals('application/json', $response->headers->get('Content-Type'));
+  }
+
+  /**
+   * @covers ::on500
+   */
+  public function testOnFatalErrorVerbose() {
+    $kernel = $this->prophesize(HttpKernelInterface::class);
+    $request = Request::create('/test');
+    $request->setRequestFormat('json');
+
+    $e = new \LogicException('Some explanation here.');
+    $event = new GetResponseForExceptionEvent($kernel->reveal(), $request, 'GET', $e);
+    $subscriber = new DefaultExceptionSubscriber(new Serializer([], [new JsonEncoder()]), ['json']);
+    $subscriber->onException($event);
+    $response = $event->getResponse();
+
+    $this->assertInstanceOf(Response::class, $response);
+    $this->assertEquals('{"message":"A fatal error occurred: Some explanation here."}', $response->getContent());
+    $this->assertEquals(500, $response->getStatusCode());
+    $this->assertEquals('application/json', $response->headers->get('Content-Type'));
+  }
+
+}
+
+}
+
+namespace {
+  if (!function_exists('error_displayable')) {
+    function error_displayable($error = NULL) {
+      if ($error['%type'] === 'LogicException') {
+        return TRUE;
+      }
+      else {
+        return FALSE;
+      }
+    }
+  }
 }