diff --git a/src/Client.php b/src/Client.php
index 77c8dfe..dfb18ba 100644
--- a/src/Client.php
+++ b/src/Client.php
@@ -22,14 +22,14 @@ class Client
 {
     private static $converter;
     private static $jsonHelper;
-    public static function getConverter(Client $client = null): ConverterInterface
+    public static function getConverter(?Client $client = null): ConverterInterface
     {
         if (isset($client)) {
             return $client->localConverter;
         }
         return self::$converter;
     }
-    public static function getJsonHelper(Client $client = null): JsonHelper
+    public static function getJsonHelper(?Client $client = null): JsonHelper
     {
         if (isset($client)) {
             return $client->localJsonHelper;
diff --git a/src/Types/Sdk/CoreCallback.php b/src/Types/Sdk/CoreCallback.php
index 7731e1a..166dbec 100644
--- a/src/Types/Sdk/CoreCallback.php
+++ b/src/Types/Sdk/CoreCallback.php
@@ -27,7 +27,7 @@ class CoreCallback
      * @param callable|null $onBeforeRequest Called before an API call
      * @param callable|null $onAfterRequest  Called after an API call
      */
-    public function __construct(callable $onBeforeRequest = null, callable $onAfterRequest = null)
+    public function __construct(?callable $onBeforeRequest = null, ?callable $onAfterRequest = null)
     {
         $this->onBeforeRequest = $onBeforeRequest;
         $this->onAfterRequest = $onAfterRequest;
diff --git a/src/Utils/XmlDeserializer.php b/src/Utils/XmlDeserializer.php
index 1e29f12..c67fc7a 100644
--- a/src/Utils/XmlDeserializer.php
+++ b/src/Utils/XmlDeserializer.php
@@ -22,7 +22,7 @@ class XmlDeserializer
     /**
      * @param int|null $loadOptions  A bit field of LIBXML_* constants
      */
-    public function __construct(int $loadOptions = null)
+    public function __construct(?int $loadOptions = null)
     {
         $this->dom = new DOMDocument();
         $this->loadOptions = $loadOptions ?? (LIBXML_NONET | LIBXML_NOBLANKS);
@@ -81,7 +81,7 @@ public function fromElementToArray(
         \DOMNode $parent,
         string $itemName,
         string $clazz,
-        string $wrappingElementName = null
+        ?string $wrappingElementName = null
     ) {
         if ($wrappingElementName === null) {
             $elements = static::getChildNodesByTagName($parent, $itemName);
diff --git a/src/Utils/XmlSerializer.php b/src/Utils/XmlSerializer.php
index 0162abe..170d936 100644
--- a/src/Utils/XmlSerializer.php
+++ b/src/Utils/XmlSerializer.php
@@ -67,7 +67,7 @@ public function addArrayAsSubelement(
         \DOMNode $root,
         string $itemName,
         $items,
-        string $wrappingElementName = null
+        ?string $wrappingElementName = null
     ): void {
         if ($items === null) {
             return;
@@ -102,7 +102,7 @@ public function addAsSubelement(\DOMNode $root, string $name, $value): ?\DOMElem
         return $element;
     }
 
-    public function createElement(string $name, string $value = null): \DOMElement
+    public function createElement(string $name, ?string $value = null): \DOMElement
     {
         return $value === null ? $this->dom->createElement($name) : $this->dom->createElement($name, $value);
     }