diff --git a/Tests/Transport/AmqpTransportTest.php b/Tests/Transport/AmqpTransportTest.php
index 433aeeb..21857d4 100644
--- a/Tests/Transport/AmqpTransportTest.php
+++ b/Tests/Transport/AmqpTransportTest.php
@@ -52,7 +52,7 @@ public function testReceivesMessages()
         $this->assertSame($decodedMessage, $envelopes[0]->getMessage());
     }
 
-    private function getTransport(SerializerInterface $serializer = null, Connection $connection = null): AmqpTransport
+    private function getTransport(?SerializerInterface $serializer = null, ?Connection $connection = null): AmqpTransport
     {
         $serializer ??= $this->createMock(SerializerInterface::class);
         $connection ??= $this->createMock(Connection::class);
diff --git a/Transport/AmqpReceiver.php b/Transport/AmqpReceiver.php
index fecf39b..631f79d 100644
--- a/Transport/AmqpReceiver.php
+++ b/Transport/AmqpReceiver.php
@@ -30,7 +30,7 @@ class AmqpReceiver implements QueueReceiverInterface, MessageCountAwareInterface
     private SerializerInterface $serializer;
     private Connection $connection;
 
-    public function __construct(Connection $connection, SerializerInterface $serializer = null)
+    public function __construct(Connection $connection, ?SerializerInterface $serializer = null)
     {
         $this->connection = $connection;
         $this->serializer = $serializer ?? new PhpSerializer();
diff --git a/Transport/AmqpSender.php b/Transport/AmqpSender.php
index cbcecff..4f7caaa 100644
--- a/Transport/AmqpSender.php
+++ b/Transport/AmqpSender.php
@@ -29,7 +29,7 @@ class AmqpSender implements SenderInterface
     private SerializerInterface $serializer;
     private Connection $connection;
 
-    public function __construct(Connection $connection, SerializerInterface $serializer = null)
+    public function __construct(Connection $connection, ?SerializerInterface $serializer = null)
     {
         $this->connection = $connection;
         $this->serializer = $serializer ?? new PhpSerializer();
diff --git a/Transport/AmqpStamp.php b/Transport/AmqpStamp.php
index 127593d..a91467f 100644
--- a/Transport/AmqpStamp.php
+++ b/Transport/AmqpStamp.php
@@ -24,7 +24,7 @@ final class AmqpStamp implements NonSendableStampInterface
     private array $attributes;
     private bool $isRetryAttempt = false;
 
-    public function __construct(string $routingKey = null, int $flags = \AMQP_NOPARAM, array $attributes = [])
+    public function __construct(?string $routingKey = null, int $flags = \AMQP_NOPARAM, array $attributes = [])
     {
         $this->routingKey = $routingKey;
         $this->flags = $flags;
@@ -46,7 +46,7 @@ public function getAttributes(): array
         return $this->attributes;
     }
 
-    public static function createFromAmqpEnvelope(\AMQPEnvelope $amqpEnvelope, self $previousStamp = null, string $retryRoutingKey = null): self
+    public static function createFromAmqpEnvelope(\AMQPEnvelope $amqpEnvelope, ?self $previousStamp = null, ?string $retryRoutingKey = null): self
     {
         $attr = $previousStamp->attributes ?? [];
 
@@ -79,7 +79,7 @@ public function isRetryAttempt(): bool
         return $this->isRetryAttempt;
     }
 
-    public static function createWithAttributes(array $attributes, self $previousStamp = null): self
+    public static function createWithAttributes(array $attributes, ?self $previousStamp = null): self
     {
         return new self(
             $previousStamp->routingKey ?? null,
diff --git a/Transport/AmqpTransport.php b/Transport/AmqpTransport.php
index 2276536..edb0961 100644
--- a/Transport/AmqpTransport.php
+++ b/Transport/AmqpTransport.php
@@ -29,7 +29,7 @@ class AmqpTransport implements QueueReceiverInterface, TransportInterface, Setup
     private AmqpReceiver $receiver;
     private AmqpSender $sender;
 
-    public function __construct(Connection $connection, SerializerInterface $serializer = null)
+    public function __construct(Connection $connection, ?SerializerInterface $serializer = null)
     {
         $this->connection = $connection;
         $this->serializer = $serializer ?? new PhpSerializer();
diff --git a/Transport/Connection.php b/Transport/Connection.php
index 260e226..fc5e5d0 100644
--- a/Transport/Connection.php
+++ b/Transport/Connection.php
@@ -97,7 +97,7 @@ class Connection
     private \AMQPExchange $amqpDelayExchange;
     private int $lastActivityTime = 0;
 
-    public function __construct(array $connectionOptions, array $exchangeOptions, array $queuesOptions, AmqpFactory $amqpFactory = null)
+    public function __construct(array $connectionOptions, array $exchangeOptions, array $queuesOptions, ?AmqpFactory $amqpFactory = null)
     {
         if (!\extension_loaded('amqp')) {
             throw new LogicException(sprintf('You cannot use the "%s" as the "amqp" extension is not installed.', __CLASS__));
@@ -161,7 +161,7 @@ public function __construct(array $connectionOptions, array $exchangeOptions, ar
      *     * verify: Enable or disable peer verification. If peer verification is enabled then the common name in the
      *       server certificate must match the server name. Peer verification is enabled by default.
      */
-    public static function fromDsn(#[\SensitiveParameter] string $dsn, array $options = [], AmqpFactory $amqpFactory = null): self
+    public static function fromDsn(#[\SensitiveParameter] string $dsn, array $options = [], ?AmqpFactory $amqpFactory = null): self
     {
         if (false === $params = parse_url($dsn)) {
             // this is a valid URI that parse_url cannot handle when you want to pass all parameters as options
@@ -279,7 +279,7 @@ private static function hasCaCertConfigured(array $amqpOptions): bool
     /**
      * @throws \AMQPException
      */
-    public function publish(string $body, array $headers = [], int $delayInMs = 0, AmqpStamp $amqpStamp = null): void
+    public function publish(string $body, array $headers = [], int $delayInMs = 0, ?AmqpStamp $amqpStamp = null): void
     {
         $this->clearWhenDisconnected();
 
@@ -313,7 +313,7 @@ public function countMessagesInQueues(): int
     /**
      * @throws \AMQPException
      */
-    private function publishWithDelay(string $body, array $headers, int $delay, AmqpStamp $amqpStamp = null): void
+    private function publishWithDelay(string $body, array $headers, int $delay, ?AmqpStamp $amqpStamp = null): void
     {
         $routingKey = $this->getRoutingKeyForMessage($amqpStamp);
         $isRetryAttempt = $amqpStamp ? $amqpStamp->isRetryAttempt() : false;
@@ -329,7 +329,7 @@ private function publishWithDelay(string $body, array $headers, int $delay, Amqp
         );
     }
 
-    private function publishOnExchange(\AMQPExchange $exchange, string $body, string $routingKey = null, array $headers = [], AmqpStamp $amqpStamp = null): void
+    private function publishOnExchange(\AMQPExchange $exchange, string $body, ?string $routingKey = null, array $headers = [], ?AmqpStamp $amqpStamp = null): void
     {
         $attributes = $amqpStamp ? $amqpStamp->getAttributes() : [];
         $attributes['headers'] = array_merge($attributes['headers'] ?? [], $headers);