diff --git a/core/modules/media/src/Controller/OEmbedIframeController.php b/core/modules/media/src/Controller/OEmbedIframeController.php
index dc98339..a32533f 100644
--- a/core/modules/media/src/Controller/OEmbedIframeController.php
+++ b/core/modules/media/src/Controller/OEmbedIframeController.php
@@ -134,7 +134,9 @@ public function render(Request $request) {
     // Return a response instead of a render array so that the frame content
     // will not have all the blocks and page elements normally rendered by
     // Drupal.
-    $response = new HtmlResponse();
+    $response = new HtmlResponse('', HtmlResponse::HTTP_OK, [
+      'Content-Type' => 'text/html; charset=UTF-8',
+    ]);
     $response->addCacheableDependency(Url::createFromRequest($request));
 
     try {
diff --git a/core/modules/media/tests/src/Kernel/OEmbedIframeControllerTest.php b/core/modules/media/tests/src/Kernel/OEmbedIframeControllerTest.php
index 0bc8c89..cc96340 100644
--- a/core/modules/media/tests/src/Kernel/OEmbedIframeControllerTest.php
+++ b/core/modules/media/tests/src/Kernel/OEmbedIframeControllerTest.php
@@ -94,15 +94,16 @@ public function testResourcePassedToPreprocess() {
       'url' => '',
       'hash' => $hash,
     ]);
-    $response = $this->container->get('html_response.attachments_processor')
-      ->processAttachments(OEmbedIframeController::create($this->container)
-        ->render($request));
+    $response = OEmbedIframeController::create($this->container)
+      ->render($request);
+    $content = $response->getContent();
     assert($response instanceof HtmlResponse);
     $content = $response->getContent();
 
     // This query parameter is added by
     // media_test_oembed_preprocess_media_oembed_iframe() for YouTube videos.
     $this->assertStringContainsString('&pasta=rigatoni', $content);
+    $this->assertStringContainsString('text/html', $response->headers->get('Content-Type'));
     $this->assertStringContainsString('test.css', $content);
     $this->assertContains('yo_there', $response->getCacheableMetadata()->getCacheTags());
   }