diff --git a/src/Common/AbstractGateway.php b/src/Common/AbstractGateway.php
index 17de242a..33443360 100755
--- a/src/Common/AbstractGateway.php
+++ b/src/Common/AbstractGateway.php
@@ -64,7 +64,7 @@ abstract class AbstractGateway implements GatewayInterface
      * @param ClientInterface          $httpClient  A HTTP client to make API calls with
      * @param HttpRequest     $httpRequest A Symfony HTTP request object
      */
-    public function __construct(ClientInterface $httpClient = null, HttpRequest $httpRequest = null)
+    public function __construct(?ClientInterface $httpClient = null, ?HttpRequest $httpRequest = null)
     {
         $this->httpClient = $httpClient ?: $this->getDefaultHttpClient();
         $this->httpRequest = $httpRequest ?: $this->getDefaultHttpRequest();
diff --git a/src/Common/CreditCard.php b/src/Common/CreditCard.php
index c92285ba..45a08472 100644
--- a/src/Common/CreditCard.php
+++ b/src/Common/CreditCard.php
@@ -190,7 +190,7 @@ public function addSupportedBrand($name, $expression)
      * @param array $parameters An associative array of parameters
      * @return $this
      */
-    public function initialize(array $parameters = null)
+    public function initialize(?array $parameters = null)
     {
         $this->parameters = new ParameterBag;
 
diff --git a/src/Common/GatewayFactory.php b/src/Common/GatewayFactory.php
index 0b5ff250..8d887d59 100644
--- a/src/Common/GatewayFactory.php
+++ b/src/Common/GatewayFactory.php
@@ -77,7 +77,7 @@ public function register($className)
      * @throws RuntimeException                 If no such gateway is found
      * @return GatewayInterface                 An object of class $class is created and returned
      */
-    public function create($class, ClientInterface $httpClient = null, HttpRequest $httpRequest = null)
+    public function create($class, ?ClientInterface $httpClient = null, ?HttpRequest $httpRequest = null)
     {
         $class = Helper::getGatewayClassName($class);
 
diff --git a/src/Common/Http/Client.php b/src/Common/Http/Client.php
index abd4a53d..e20a6106 100644
--- a/src/Common/Http/Client.php
+++ b/src/Common/Http/Client.php
@@ -29,7 +29,7 @@ class Client implements ClientInterface
      */
     private $requestFactory;
 
-    public function __construct($httpClient = null, RequestFactory $requestFactory = null)
+    public function __construct($httpClient = null, ?RequestFactory $requestFactory = null)
     {
         $this->httpClient = $httpClient ?: HttpClientDiscovery::find();
         $this->requestFactory = $requestFactory ?: MessageFactoryDiscovery::find();
diff --git a/src/Common/Item.php b/src/Common/Item.php
index 7279abc4..870ee528 100644
--- a/src/Common/Item.php
+++ b/src/Common/Item.php
@@ -22,7 +22,7 @@ class Item implements ItemInterface
      *
      * @param array|null $parameters An array of parameters to set on the new object
      */
-    public function __construct(array $parameters = null)
+    public function __construct(?array $parameters = null)
     {
         $this->initialize($parameters);
     }
@@ -33,7 +33,7 @@ public function __construct(array $parameters = null)
      * @param array|null $parameters An array of parameters to set on this object
      * @return $this Item
      */
-    public function initialize(array $parameters = null)
+    public function initialize(?array $parameters = null)
     {
         $this->parameters = new ParameterBag;
 
diff --git a/src/Omnipay.php b/src/Omnipay.php
index d3bf7fb0..85903958 100644
--- a/src/Omnipay.php
+++ b/src/Omnipay.php
@@ -83,7 +83,7 @@ public static function getFactory()
      *
      * @param GatewayFactory $factory A GatewayFactory instance
      */
-    public static function setFactory(GatewayFactory $factory = null)
+    public static function setFactory(?GatewayFactory $factory = null)
     {
         self::$factory = $factory;
     }