core/lib/Drupal/Core/Routing/RequestFormatRouteFilter.php          | 3 +++
 .../tests/src/Functional/EntityResource/EntityResourceTestBase.php | 7 +++----
 core/tests/Drupal/FunctionalTests/Routing/DefaultFormatTest.php    | 2 +-
 3 files changed, 7 insertions(+), 5 deletions(-)

diff --git a/core/lib/Drupal/Core/Routing/RequestFormatRouteFilter.php b/core/lib/Drupal/Core/Routing/RequestFormatRouteFilter.php
index dbfb801..1586db8 100644
--- a/core/lib/Drupal/Core/Routing/RequestFormatRouteFilter.php
+++ b/core/lib/Drupal/Core/Routing/RequestFormatRouteFilter.php
@@ -18,6 +18,9 @@ public function filter(RouteCollection $collection, Request $request) {
     // Determine the request format.
     $default_format = static::getDefaultFormat($collection);
     $format = $request->getRequestFormat($default_format);
+    if (is_null($request->getRequestFormat(NULL))) {
+      $request->setRequestFormat($format);
+    }
 
     $routes_with_requirement = [];
     $routes_without_requirement = [];
diff --git a/core/modules/rest/tests/src/Functional/EntityResource/EntityResourceTestBase.php b/core/modules/rest/tests/src/Functional/EntityResource/EntityResourceTestBase.php
index 257c580..8f48a0c 100644
--- a/core/modules/rest/tests/src/Functional/EntityResource/EntityResourceTestBase.php
+++ b/core/modules/rest/tests/src/Functional/EntityResource/EntityResourceTestBase.php
@@ -470,17 +470,16 @@ public function testGet() {
 
     // First: single format. Drupal will automatically pick the only format.
     $this->provisionEntityResource(TRUE);
+    $expected_403_cacheability = $this->getExpectedUnauthorizedAccessCacheability();
     // DX: 403 because unauthorized single-format route, ?_format is omittable.
     $url->setOption('query', []);
     $response = $this->request('GET', $url, $request_options);
-    $this->assertSame(403, $response->getStatusCode());
-    $this->assertSame(['text/html; charset=UTF-8'], $response->getHeader('Content-Type'));
+    $this->assertResourceErrorResponse(403, FALSE, $response, $expected_403_cacheability->getCacheTags(), $expected_403_cacheability->getCacheContexts(), static::$auth ? FALSE : 'MISS', 'MISS');
     $this->assertSame(static::$auth ? [] : ['MISS'], $response->getHeader('X-Drupal-Cache'));
     // DX: 403 because unauthorized.
     $url->setOption('query', ['_format' => static::$format]);
     $response = $this->request('GET', $url, $request_options);
-    $expected_403_cacheability = $this->getExpectedUnauthorizedAccessCacheability();
-    $this->assertResourceErrorResponse(403, FALSE, $response, $expected_403_cacheability->getCacheTags(), $expected_403_cacheability->getCacheContexts(), static::$auth ? FALSE : 'MISS', 'MISS');
+    $this->assertResourceErrorResponse(403, FALSE, $response, $expected_403_cacheability->getCacheTags(), $expected_403_cacheability->getCacheContexts(), static::$auth ? FALSE : 'MISS', 'HIT');
 
     // Then, what we'll use for the remainder of the test: multiple formats.
     $this->provisionEntityResource();
diff --git a/core/tests/Drupal/FunctionalTests/Routing/DefaultFormatTest.php b/core/tests/Drupal/FunctionalTests/Routing/DefaultFormatTest.php
index fba590e..7a05991 100644
--- a/core/tests/Drupal/FunctionalTests/Routing/DefaultFormatTest.php
+++ b/core/tests/Drupal/FunctionalTests/Routing/DefaultFormatTest.php
@@ -20,7 +20,7 @@ public function testFoo() {
     $this->assertSame('MISS', $this->drupalGetHeader('X-Drupal-Cache'));
 
     $this->drupalGet('/default_format_test/machine');
-    $this->assertSame('format:html', $this->getSession()->getPage()->getContent());
+    $this->assertSame('format:json', $this->getSession()->getPage()->getContent());
     $this->assertSame('MISS', $this->drupalGetHeader('X-Drupal-Cache'));
   }