diff --git a/Tests/Transport/BeanstalkdTransportTest.php b/Tests/Transport/BeanstalkdTransportTest.php
index 5671163..19184ed 100644
--- a/Tests/Transport/BeanstalkdTransportTest.php
+++ b/Tests/Transport/BeanstalkdTransportTest.php
@@ -50,7 +50,7 @@ public function testReceivesMessages()
         $this->assertSame($decodedMessage, $envelopes[0]->getMessage());
     }
 
-    private function getTransport(SerializerInterface $serializer = null, Connection $connection = null): BeanstalkdTransport
+    private function getTransport(?SerializerInterface $serializer = null, ?Connection $connection = null): BeanstalkdTransport
     {
         $serializer = $serializer ?? $this->createMock(SerializerInterface::class);
         $connection = $connection ?? $this->createMock(Connection::class);
diff --git a/Transport/BeanstalkdReceiver.php b/Transport/BeanstalkdReceiver.php
index 0a5ca05..f5415ae 100644
--- a/Transport/BeanstalkdReceiver.php
+++ b/Transport/BeanstalkdReceiver.php
@@ -27,7 +27,7 @@ class BeanstalkdReceiver implements ReceiverInterface, MessageCountAwareInterfac
     private $connection;
     private $serializer;
 
-    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/BeanstalkdSender.php b/Transport/BeanstalkdSender.php
index 58f02dc..48b11a8 100644
--- a/Transport/BeanstalkdSender.php
+++ b/Transport/BeanstalkdSender.php
@@ -25,7 +25,7 @@ class BeanstalkdSender implements SenderInterface
     private $connection;
     private $serializer;
 
-    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/BeanstalkdTransport.php b/Transport/BeanstalkdTransport.php
index 480d6e3..9a06808 100644
--- a/Transport/BeanstalkdTransport.php
+++ b/Transport/BeanstalkdTransport.php
@@ -27,7 +27,7 @@ class BeanstalkdTransport implements TransportInterface, MessageCountAwareInterf
     private $receiver;
     private $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();