diff --git a/lib/Api/TransactionalEmailsApi.php b/lib/Api/TransactionalEmailsApi.php
index ceca9c1..0ab2757 100644
--- a/lib/Api/TransactionalEmailsApi.php
+++ b/lib/Api/TransactionalEmailsApi.php
@@ -70,9 +70,9 @@ class TransactionalEmailsApi
      * @param HeaderSelector  $selector
      */
     public function __construct(
-        ClientInterface $client = null,
-        Configuration $config = null,
-        HeaderSelector $selector = null
+        ?ClientInterface $client = null,
+        ?Configuration $config = null,
+        ?HeaderSelector $selector = null
     ) {
         $this->client = $client ?: new Client();
         $this->config = $config ?: new Configuration();
diff --git a/lib/Model/SendSmtpEmail.php b/lib/Model/SendSmtpEmail.php
index 58386de..4a8ff39 100644
--- a/lib/Model/SendSmtpEmail.php
+++ b/lib/Model/SendSmtpEmail.php
@@ -250,7 +250,7 @@ public function getModelName()
      * @param mixed[] $data Associated array of property values
      *                      initializing the model
      */
-    public function __construct(array $data = null)
+    public function __construct(?array $data = null)
     {
         $this->container['sender'] = isset($data['sender']) ? $data['sender'] : null;
         $this->container['to'] = isset($data['to']) ? $data['to'] : null;
diff --git a/lib/Model/SendSmtpEmailAttachment.php b/lib/Model/SendSmtpEmailAttachment.php
index 4775f9c..f8fdf0a 100644
--- a/lib/Model/SendSmtpEmailAttachment.php
+++ b/lib/Model/SendSmtpEmailAttachment.php
@@ -185,7 +185,7 @@ public function getModelName()
      * @param mixed[] $data Associated array of property values
      *                      initializing the model
      */
-    public function __construct(array $data = null)
+    public function __construct(?array $data = null)
     {
         $this->container['url'] = isset($data['url']) ? $data['url'] : null;
         $this->container['content'] = isset($data['content']) ? $data['content'] : null;
diff --git a/lib/Model/SendSmtpEmailReplyTo.php b/lib/Model/SendSmtpEmailReplyTo.php
index e5231f7..d69e046 100644
--- a/lib/Model/SendSmtpEmailReplyTo.php
+++ b/lib/Model/SendSmtpEmailReplyTo.php
@@ -181,7 +181,7 @@ public function getModelName()
      * @param mixed[] $data Associated array of property values
      *                      initializing the model
      */
-    public function __construct(array $data = null)
+    public function __construct(?array $data = null)
     {
         $this->container['email'] = isset($data['email']) ? $data['email'] : null;
         $this->container['name'] = isset($data['name']) ? $data['name'] : null;
diff --git a/lib/Model/SendSmtpEmailSender.php b/lib/Model/SendSmtpEmailSender.php
index b27b168..db7d6c8 100644
--- a/lib/Model/SendSmtpEmailSender.php
+++ b/lib/Model/SendSmtpEmailSender.php
@@ -186,7 +186,7 @@ public function getModelName()
      * @param mixed[] $data Associated array of property values
      *                      initializing the model
      */
-    public function __construct(array $data = null)
+    public function __construct(?array $data = null)
     {
         $this->container['name'] = isset($data['name']) ? $data['name'] : null;
         $this->container['email'] = isset($data['email']) ? $data['email'] : null;
diff --git a/lib/Model/SendSmtpEmailTo.php b/lib/Model/SendSmtpEmailTo.php
index e7c164b..17ca1d4 100644
--- a/lib/Model/SendSmtpEmailTo.php
+++ b/lib/Model/SendSmtpEmailTo.php
@@ -180,7 +180,7 @@ public function getModelName()
      * @param mixed[] $data Associated array of property values
      *                      initializing the model
      */
-    public function __construct(array $data = null)
+    public function __construct(?array $data = null)
     {
         $this->container['email'] = isset($data['email']) ? $data['email'] : null;
         $this->container['name'] = isset($data['name']) ? $data['name'] : null;