.../EntityResource/EntityResourceTestBase.php        |  2 +-
 .../EventSubscriber/DefaultExceptionSubscriber.php   | 20 ++++++++++++--------
 2 files changed, 13 insertions(+), 9 deletions(-)

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'));
diff --git a/core/modules/serialization/src/EventSubscriber/DefaultExceptionSubscriber.php b/core/modules/serialization/src/EventSubscriber/DefaultExceptionSubscriber.php
index 5a21bf4..b157f62 100644
--- a/core/modules/serialization/src/EventSubscriber/DefaultExceptionSubscriber.php
+++ b/core/modules/serialization/src/EventSubscriber/DefaultExceptionSubscriber.php
@@ -63,16 +63,20 @@ protected static function getPriority() {
    */
   public function onException(GetResponseForExceptionEvent $event) {
     $exception = $event->getException();
-    if (in_array($event->getRequest()->getRequestFormat(), $this->getHandledFormats(), TRUE)) {
-      if (!$exception instanceof HttpExceptionInterface) {
-        $error = Error::decodeException($exception);
-        $message = error_displayable($error)
-          ? sprintf('A fatal error occurred: %s', $exception->getMessage())
-          : 'A fatal error occurred.';
-        $event->setException(new HttpException(500, $message, $exception));
-      }
+
+    // Map non-HttpExceptionInterface exceptions for fatal errors onto the same
+    // HttpExceptionInterface object, so we can handle them consistently.
+    if (!$exception instanceof HttpExceptionInterface && in_array($event->getRequest()->getRequestFormat(), $this->getHandledFormats(), TRUE)) {
+      $error = Error::decodeException($exception);
+      $message = error_displayable($error)
+        ? sprintf('A fatal error occurred: %s', $exception->getMessage())
+        : 'A fatal error occurred.';
+      $event->setException(new HttpException(500, $message, $exception));
     }
+
     parent::onException($event);
+
+    // If we didn't generate an error response, restore the original exception.
     if (!$event->isPropagationStopped()) {
       $event->setException($exception);
     }