diff --git a/src/Factory.php b/src/Factory.php
index f5b9899..49082c7 100644
--- a/src/Factory.php
+++ b/src/Factory.php
@@ -63,7 +63,7 @@ class Factory
      * @return T
      * @throws ClassDoesNotExistsException
      */
-    public static function create($class, Authentication $authentication = null)
+    public static function create($class, ?Authentication $authentication = null)
     {
         if (class_exists($class)) {
             return new $class($authentication);
diff --git a/src/Request/Customer.php b/src/Request/Customer.php
index f04cb8e..2b98747 100644
--- a/src/Request/Customer.php
+++ b/src/Request/Customer.php
@@ -267,7 +267,7 @@ class Customer extends AbstractSerializer
      *
      * @param Address|null $billingAddress Billing address
      */
-    public function __construct(Address $billingAddress = null)
+    public function __construct(?Address $billingAddress = null)
     {
         $this->billing = $billingAddress;
     }
diff --git a/src/Response/AbstractResponse.php b/src/Response/AbstractResponse.php
index c15783c..d6fac30 100644
--- a/src/Response/AbstractResponse.php
+++ b/src/Response/AbstractResponse.php
@@ -49,7 +49,7 @@ abstract class AbstractResponse
      * @return void
      * @throws \InvalidArgumentException
      */
-    public function headerSetter(\SimpleXMLElement $xml = null)
+    public function headerSetter(?\SimpleXMLElement $xml = null)
     {
         if ($xml) {
             $this->Header = ResponseSerializer::serialize(Header::class, $xml);
@@ -64,7 +64,7 @@ public function headerSetter(\SimpleXMLElement $xml = null)
      * @return static
      * @throws \InvalidArgumentException
      */
-    public function deserialize(\SimpleXMLElement $xml = null)
+    public function deserialize(?\SimpleXMLElement $xml = null)
     {
         $object = clone $this;
 
diff --git a/src/Serializer/ResponseSerializer.php b/src/Serializer/ResponseSerializer.php
index 7403e27..ecb0908 100644
--- a/src/Serializer/ResponseSerializer.php
+++ b/src/Serializer/ResponseSerializer.php
@@ -45,7 +45,7 @@ class ResponseSerializer
     public static function serialize(
         $objectName,
         \SimpleXMLElement $data,
-        \SimpleXMLElement $header = null
+        ?\SimpleXMLElement $header = null
     ) {
         $object = new $objectName();
         $object->headerSetter($header);