diff --git a/src/Api/Domain.php b/src/Api/Domain.php
index 44274419..73214553 100644
--- a/src/Api/Domain.php
+++ b/src/Api/Domain.php
@@ -101,10 +101,10 @@ public function show(string $domain, array $requestHeaders = [])
      */
     public function create(
         string $domain,
-        string $smtpPass = null,
-        string $spamAction = null,
-        bool $wildcard = null,
-        bool $forceDkimAuthority = null,
+        ?string $smtpPass = null,
+        ?string $spamAction = null,
+        ?bool $wildcard = null,
+        ?bool $forceDkimAuthority = null,
         ?array $ips = null,
         ?string $pool_id = null,
         string $webScheme = 'http',
diff --git a/src/Api/DomainV4.php b/src/Api/DomainV4.php
index 7b530c77..e80f416c 100644
--- a/src/Api/DomainV4.php
+++ b/src/Api/DomainV4.php
@@ -99,8 +99,8 @@ public function show(string $domain, array $requestHeaders = [])
      */
     public function create(
         string  $domain,
-        string  $smtpPass = null,
-        string  $spamAction = null,
+        ?string  $smtpPass = null,
+        ?string  $spamAction = null,
         ?bool $wildcard = null,
         ?bool $forceDkimAuthority = null,
         ?array  $ips = null,
diff --git a/src/Api/MailingList/Member.php b/src/Api/MailingList/Member.php
index ee9c725f..e0680ed7 100644
--- a/src/Api/MailingList/Member.php
+++ b/src/Api/MailingList/Member.php
@@ -39,7 +39,7 @@ class Member extends HttpApi
      * @return IndexResponse
      * @throws ClientExceptionInterface
      */
-    public function index(string $address, int $limit = 100, bool $subscribed = null, array $requestHeaders = [])
+    public function index(string $address, int $limit = 100, ?bool $subscribed = null, array $requestHeaders = [])
     {
         Assert::stringNotEmpty($address);
         Assert::greaterThan($limit, 0);
@@ -92,7 +92,7 @@ public function show(string $list, string $address, array $requestHeaders = [])
     public function create(
         string $list,
         string $address,
-        string $name = null,
+        ?string $name = null,
         array $vars = [],
         bool $subscribed = true,
         bool $upsert = false,
diff --git a/src/Api/Route.php b/src/Api/Route.php
index bcf28702..64c76479 100644
--- a/src/Api/Route.php
+++ b/src/Api/Route.php
@@ -107,10 +107,10 @@ public function create(string $expression, array $actions, string $description,
      */
     public function update(
         string $routeId,
-        string $expression = null,
+        ?string $expression = null,
         array $actions = [],
-        string $description = null,
-        int $priority = null,
+        ?string $description = null,
+        ?int $priority = null,
         array $requestHeaders = []
     ) {
         Assert::stringNotEmpty($routeId);
diff --git a/src/Api/Suppression/Complaint.php b/src/Api/Suppression/Complaint.php
index 4e25121c..18b6014e 100644
--- a/src/Api/Suppression/Complaint.php
+++ b/src/Api/Suppression/Complaint.php
@@ -73,7 +73,7 @@ public function show(string $domain, string $address, array $requestHeaders = []
      * @return CreateResponse
      * @throws ClientExceptionInterface
      */
-    public function create(string $domain, string $address, string $createdAt = null, array $requestHeaders = [])
+    public function create(string $domain, string $address, ?string $createdAt = null, array $requestHeaders = [])
     {
         Assert::stringNotEmpty($domain);
         Assert::stringNotEmpty($address);
diff --git a/src/Api/Suppression/Unsubscribe.php b/src/Api/Suppression/Unsubscribe.php
index 646470fb..54cd93dd 100644
--- a/src/Api/Suppression/Unsubscribe.php
+++ b/src/Api/Suppression/Unsubscribe.php
@@ -95,7 +95,7 @@ public function create(string $domain, string $address, array $params = [], arra
      * @return DeleteResponse
      * @throws ClientExceptionInterface
      */
-    public function delete(string $domain, string $address, string $tag = null, array $requestHeaders = [])
+    public function delete(string $domain, string $address, ?string $tag = null, array $requestHeaders = [])
     {
         Assert::stringNotEmpty($domain);
         Assert::stringNotEmpty($address);
diff --git a/src/Mailgun.php b/src/Mailgun.php
index 7821320e..ba381165 100644
--- a/src/Mailgun.php
+++ b/src/Mailgun.php
@@ -77,8 +77,8 @@ class Mailgun
      */
     public function __construct(
         HttpClientConfigurator $configurator,
-        Hydrator $hydrator = null,
-        RequestBuilder $requestBuilder = null
+        ?Hydrator $hydrator = null,
+        ?RequestBuilder $requestBuilder = null
     ) {
         $this->requestBuilder = $requestBuilder ?: new RequestBuilder();
         $this->hydrator = $hydrator ?: new ModelHydrator();
diff --git a/src/Message/Exceptions/MissingRequiredParameter.php b/src/Message/Exceptions/MissingRequiredParameter.php
index 02104aff..33d2e770 100644
--- a/src/Message/Exceptions/MissingRequiredParameter.php
+++ b/src/Message/Exceptions/MissingRequiredParameter.php
@@ -20,7 +20,7 @@ class MissingRequiredParameter extends \Exception implements Exception
      * @param string|null $message
      * @return self
      */
-    public static function create(string $parameter, string $message = null)
+    public static function create(string $parameter, ?string $message = null)
     {
         if (null === $message) {
             $message = 'The parameters passed to the API were invalid. Please specify "%s".';
diff --git a/src/Message/MessageBuilder.php b/src/Message/MessageBuilder.php
index 2063d7f2..6c3d2e0e 100644
--- a/src/Message/MessageBuilder.php
+++ b/src/Message/MessageBuilder.php
@@ -318,7 +318,7 @@ public function setHtmlBody(string $htmlBody): self
      * @param  string|null $attachmentName
      * @return $this
      */
-    public function addAttachment(string $attachmentPath, string $attachmentName = null): self
+    public function addAttachment(string $attachmentPath, ?string $attachmentName = null): self
     {
         if (!isset($this->message['attachment'])) {
             $this->message['attachment'] = [];
@@ -337,7 +337,7 @@ public function addAttachment(string $attachmentPath, string $attachmentName = n
      * @param  string|null $attachmentName
      * @return $this
      */
-    public function addStringAttachment(string $attachmentContent, string $attachmentName = null): self
+    public function addStringAttachment(string $attachmentContent, ?string $attachmentName = null): self
     {
         if (!isset($this->message['attachment'])) {
             $this->message['attachment'] = [];
@@ -356,7 +356,7 @@ public function addStringAttachment(string $attachmentContent, string $attachmen
      * @param  string|null $inlineImageName
      * @return $this
      */
-    public function addInlineImage(string $inlineImagePath, string $inlineImageName = null): self
+    public function addInlineImage(string $inlineImagePath, ?string $inlineImageName = null): self
     {
         if (!isset($this->message['inline'])) {
             $this->message['inline'] = [];
@@ -466,7 +466,7 @@ public function setClickTracking(bool $enabled, bool $htmlOnly = false): self
      * @return $this
      * @throws \Exception
      */
-    public function setDeliveryTime(string $timeDate, string $timeZone = null): self
+    public function setDeliveryTime(string $timeDate, ?string $timeZone = null): self
     {
         if (null !== $timeZone) {
             $timeZoneObj = new DateTimeZone($timeZone);