diff --git a/src/Sender/RequestBuilder.php b/src/Sender/RequestBuilder.php
index 6ee13e8..b6a8803 100644
--- a/src/Sender/RequestBuilder.php
+++ b/src/Sender/RequestBuilder.php
@@ -31,7 +31,7 @@ public function createRequest(
     ): RequestInterface {
         $request = new Request('POST', $subscription->getEndpoint());
         $request = $this->withOptionalHeaders($request, $message);
-        $request = $request->withHeader('TTL', $ttl);
+        $request = $request->withHeader('TTL', (string) $ttl);
 
         if (null !== $message->getPayload() && null !== $subscription->getPublicKey() && null !== $subscription->getAuthToken()) {
             $request = $request
@@ -57,11 +57,11 @@ public function createRequest(
 
             return $request
                 ->withBody(GuzzleUtils::streamFor($content))
-                ->withHeader('Content-Length', Utils::safeStrlen($content));
+                ->withHeader('Content-Length', (string) Utils::safeStrlen($content));
         }
 
         return $request
-            ->withHeader('Content-Length', 0);
+            ->withHeader('Content-Length', '0');
     }
 
     /**
@@ -85,8 +85,7 @@ public function withVAPIDAuthentication(RequestInterface $request, array $vapid,
             if ($request->hasHeader('Crypto-Key')) {
                 $request = $request->withHeader('Crypto-Key', $request->getHeaderLine('Crypto-Key').';'.$vapidHeaders['Crypto-Key']);
             } else {
-                $headers['Crypto-Key'] = $vapidHeaders['Crypto-Key'];
-                $request->withHeader('Crypto-Key', $vapidHeaders['Crypto-Key']);
+                $request = $request->withHeader('Crypto-Key', $vapidHeaders['Crypto-Key']);
             }
         } elseif ('aes128gcm' === $subscription->getContentEncoding() && self::FCM_BASE_URL === substr($endpoint, 0, strlen(self::FCM_BASE_URL))) {
             $request = $request->withUri(new Uri(str_replace('fcm/send', 'wp', $endpoint)));
@@ -110,7 +109,7 @@ private function withOptionalHeaders(RequestInterface $request, PushMessage $mes
     {
         foreach (['urgency', 'topic'] as $option) {
             if (null !== $message->getOption($option)) {
-                $request = $request->withHeader($option, $message->getOption($option));
+                $request = $request->withHeader($option, (string) $message->getOption($option));
             }
         }
 
diff --git a/tests/Sender/RequestBuilderTest.php b/tests/Sender/RequestBuilderTest.php
new file mode 100644
index 0000000..813d3d9
--- /dev/null
+++ b/tests/Sender/RequestBuilderTest.php
@@ -0,0 +1,116 @@
+<?php
+
+namespace BenTools\WebPushBundle\Tests\Sender;
+
+use BenTools\WebPushBundle\Model\Message\PushMessage;
+use BenTools\WebPushBundle\Sender\RequestBuilder;
+use BenTools\WebPushBundle\Tests\Classes\TestUser;
+use BenTools\WebPushBundle\Tests\Classes\TestUserSubscription;
+use Minishlink\WebPush\VAPID;
+use PHPUnit\Framework\TestCase;
+
+final class RequestBuilderTest extends TestCase
+{
+    private RequestBuilder $requestBuilder;
+
+    protected function setUp(): void
+    {
+        $this->requestBuilder = new RequestBuilder();
+    }
+
+    public function testCreateRequestWithoutPayload(): void
+    {
+        $user = new TestUser('test-user');
+        $subscription = new TestUserSubscription(
+            $user,
+            'https://fcm.googleapis.com/fcm/send/test-token',
+            'BNcRdreALRFXTkOOUHK1EtK2wtaz5Ry4YfYCA_0QTpQtUbVlUls0VJXg7A8u-Ts1XbjhazAkj7I99e8QcYP7DkM=',
+            'tBNxuxajfzpWUZAH-5B20A==',
+            'sub-hash'
+        );
+
+        $message = new PushMessage();
+        $message->setUrgency('high');
+        $message->setTopic('updates');
+
+        $request = $this->requestBuilder->createRequest($message, $subscription, 120);
+
+        $this->assertSame('POST', $request->getMethod());
+        $this->assertSame('https://fcm.googleapis.com/fcm/send/test-token', (string)$request->getUri());
+        $this->assertSame(['120'], $request->getHeader('TTL'));
+        $this->assertSame(['0'], $request->getHeader('Content-Length'));
+        $this->assertSame(['high'], $request->getHeader('urgency'));
+        $this->assertSame(['updates'], $request->getHeader('topic'));
+    }
+
+    public function testCreateRequestWithPayload(): void
+    {
+        $user = new TestUser('test-user');
+        $subscription = new TestUserSubscription(
+            $user,
+            'https://fcm.googleapis.com/fcm/send/test-token',
+            'BNcRdreALRFXTkOOUHK1EtK2wtaz5Ry4YfYCA_0QTpQtUbVlUls0VJXg7A8u-Ts1XbjhazAkj7I99e8QcYP7DkM=',
+            'tBNxuxajfzpWUZAH-5B20A==',
+            'sub-hash'
+        );
+
+        $message = new PushMessage('{"title":"Hello World"}');
+        $request = $this->requestBuilder->createRequest($message, $subscription, 3600);
+
+        $this->assertSame(['3600'], $request->getHeader('TTL'));
+        $this->assertSame(['application/octet-stream'], $request->getHeader('Content-Type'));
+        $this->assertSame(['aesgcm'], $request->getHeader('Content-Encoding'));
+        $this->assertTrue($request->hasHeader('Content-Length'));
+        $contentLength = $request->getHeaderLine('Content-Length');
+        $this->assertIsNumeric($contentLength);
+        $this->assertGreaterThan(0, (int)$contentLength);
+        $this->assertTrue($request->hasHeader('Encryption'));
+        $this->assertTrue($request->hasHeader('Crypto-Key'));
+    }
+
+    public function testWithVAPIDAuthentication(): void
+    {
+        $vapidKeys = VAPID::createVapidKeys();
+        $user = new TestUser('test-user');
+        $subscription = new TestUserSubscription(
+            $user,
+            'https://fcm.googleapis.com/fcm/send/test-token',
+            'BNcRdreALRFXTkOOUHK1EtK2wtaz5Ry4YfYCA_0QTpQtUbVlUls0VJXg7A8u-Ts1XbjhazAkj7I99e8QcYP7DkM=',
+            'tBNxuxajfzpWUZAH-5B20A==',
+            'sub-hash'
+        );
+
+        $message = new PushMessage();
+        $request = $this->requestBuilder->createRequest($message, $subscription, 0);
+
+        $vapid = VAPID::validate([
+            'subject' => 'mailto:admin@example.com',
+            'publicKey' => $vapidKeys['publicKey'],
+            'privateKey' => $vapidKeys['privateKey'],
+        ]);
+
+        $authenticatedRequest = $this->requestBuilder->withVAPIDAuthentication($request, $vapid, $subscription);
+
+        $this->assertTrue($authenticatedRequest->hasHeader('Authorization'));
+        $this->assertTrue($authenticatedRequest->hasHeader('Crypto-Key'));
+    }
+
+    public function testWithGCMAuthentication(): void
+    {
+        $user = new TestUser('test-user');
+        $subscription = new TestUserSubscription(
+            $user,
+            'https://fcm.googleapis.com/fcm/send/test-token',
+            'BNcRdreALRFXTkOOUHK1EtK2wtaz5Ry4YfYCA_0QTpQtUbVlUls0VJXg7A8u-Ts1XbjhazAkj7I99e8QcYP7DkM=',
+            'tBNxuxajfzpWUZAH-5B20A==',
+            'sub-hash'
+        );
+
+        $message = new PushMessage();
+        $request = $this->requestBuilder->createRequest($message, $subscription, 0);
+
+        $authenticatedRequest = $this->requestBuilder->withGCMAuthentication($request, 'sample-api-key');
+
+        $this->assertSame(['key=sample-api-key'], $authenticatedRequest->getHeader('Authorization'));
+    }
+}