.../rest/tests/modules/rest_test/rest_test.services.yml      |  4 ++++
 .../src/Functional/EntityResource/EntityResourceTestBase.php | 12 +++++++++---
 2 files changed, 13 insertions(+), 3 deletions(-)

diff --git a/core/modules/rest/tests/modules/rest_test/rest_test.services.yml b/core/modules/rest/tests/modules/rest_test/rest_test.services.yml
index d316cf6..85b418a 100644
--- a/core/modules/rest/tests/modules/rest_test/rest_test.services.yml
+++ b/core/modules/rest/tests/modules/rest_test/rest_test.services.yml
@@ -12,3 +12,7 @@ services:
       public: false
       tags:
         - { name: page_cache_request_policy }
+  rest_test.encoder.foobar:
+    class: Drupal\serialization\Encoder\JsonEncoder
+    tags:
+      - { name: encoder, format: foobar }
diff --git a/core/modules/rest/tests/src/Functional/EntityResource/EntityResourceTestBase.php b/core/modules/rest/tests/src/Functional/EntityResource/EntityResourceTestBase.php
index 3f5bcf4..9618f99 100644
--- a/core/modules/rest/tests/src/Functional/EntityResource/EntityResourceTestBase.php
+++ b/core/modules/rest/tests/src/Functional/EntityResource/EntityResourceTestBase.php
@@ -164,7 +164,7 @@ protected function provisionEntityResource($single_format = FALSE) {
     $this->resourceConfigStorage->delete($this->resourceConfigStorage->loadMultiple());
     $format = $single_format
       ? [static::$format]
-      : [static::$format, 'xml'];
+      : [static::$format, 'foobar'];
     // It's possible to not have any authentication providers enabled, when
     // testing public (anonymous) usage of a REST resource.
     $auth = isset(static::$auth) ? [static::$auth] : [];
@@ -485,10 +485,16 @@ public function testGet() {
     // DX: 406 because despite unauthorized, ?_format is not omittable.
     $url->setOption('query', []);
     $response = $this->request('GET', $url, $request_options);
-    $this->assertSame(406, $response->getStatusCode());
+    if ($has_canonical_url) {
+      $this->assertSame(403, $response->getStatusCode());
+      $this->assertSame(['HIT'], $response->getHeader('X-Drupal-Dynamic-Cache'));
+    }
+    else {
+      $this->assertSame(406, $response->getStatusCode());
+      $this->assertSame(['UNCACHEABLE'], $response->getHeader('X-Drupal-Dynamic-Cache'));
+    }
     $this->assertSame(['text/html; charset=UTF-8'], $response->getHeader('Content-Type'));
     $this->assertSame(static::$auth ? [] : ['MISS'], $response->getHeader('X-Drupal-Cache'));
-    $this->assertSame(['UNCACHEABLE'], $response->getHeader('X-Drupal-Dynamic-Cache'));
     // DX: 403 because unauthorized.
     $url->setOption('query', ['_format' => static::$format]);
     $response = $this->request('GET', $url, $request_options);