diff --git a/Tests/Transport/PostmarkApiTransportTest.php b/Tests/Transport/PostmarkApiTransportTest.php
index 7cb56fa..0b8b188 100644
--- a/Tests/Transport/PostmarkApiTransportTest.php
+++ b/Tests/Transport/PostmarkApiTransportTest.php
@@ -13,7 +13,7 @@
 
 use PHPUnit\Framework\TestCase;
 use Symfony\Component\HttpClient\MockHttpClient;
-use Symfony\Component\HttpClient\Response\MockResponse;
+use Symfony\Component\HttpClient\Response\JsonMockResponse;
 use Symfony\Component\Mailer\Bridge\Postmark\Transport\MessageStreamHeader;
 use Symfony\Component\Mailer\Bridge\Postmark\Transport\PostmarkApiTransport;
 use Symfony\Component\Mailer\Envelope;
@@ -82,7 +82,7 @@ public function testSend()
             $this->assertSame('Hello!', $body['Subject']);
             $this->assertSame('Hello There!', $body['TextBody']);
 
-            return new MockResponse(json_encode(['MessageID' => 'foobar']), [
+            return new JsonMockResponse(['MessageID' => 'foobar'], [
                 'http_code' => 200,
             ]);
         });
@@ -102,11 +102,8 @@ public function testSend()
 
     public function testSendThrowsForErrorResponse()
     {
-        $client = new MockHttpClient(static fn (string $method, string $url, array $options): ResponseInterface => new MockResponse(json_encode(['Message' => 'i\'m a teapot', 'ErrorCode' => 418]), [
+        $client = new MockHttpClient(static fn (string $method, string $url, array $options): ResponseInterface => new JsonMockResponse(['Message' => 'i\'m a teapot', 'ErrorCode' => 418], [
             'http_code' => 418,
-            'response_headers' => [
-                'content-type' => 'application/json',
-            ],
         ]));
         $transport = new PostmarkApiTransport('KEY', $client);
         $transport->setPort(8984);