diff --git a/src/Conversations/ConversationsEndpoint.php b/src/Conversations/ConversationsEndpoint.php
index 8f2640e5..b36ddff6 100644
--- a/src/Conversations/ConversationsEndpoint.php
+++ b/src/Conversations/ConversationsEndpoint.php
@@ -15,7 +15,7 @@
 
 class ConversationsEndpoint extends Endpoint
 {
-    public function get(int $id, ConversationRequest $conversationRequest = null): Conversation
+    public function get(int $id, ?ConversationRequest $conversationRequest = null): Conversation
     {
         $conversationResource = $this->restClient->getResource(Conversation::class, sprintf('/v2/conversations/%d', $id));
 
@@ -96,8 +96,8 @@ public function updateTags(int $conversationId, $tags): void
      * @return Conversation[]|PagedCollection
      */
     public function list(
-        ConversationFilters $conversationFilters = null,
-        ConversationRequest $conversationRequest = null
+        ?ConversationFilters $conversationFilters = null,
+        ?ConversationRequest $conversationRequest = null
     ): PagedCollection {
         $uri = '/v2/conversations';
         if ($conversationFilters) {
diff --git a/src/Conversations/CustomerWaitingSince.php b/src/Conversations/CustomerWaitingSince.php
index 1d04c7dc..9500c892 100644
--- a/src/Conversations/CustomerWaitingSince.php
+++ b/src/Conversations/CustomerWaitingSince.php
@@ -55,7 +55,7 @@ public function extract(): array
         return $fields;
     }
 
-    public function setTime(DateTime $time = null): self
+    public function setTime(?DateTime $time = null): self
     {
         $this->time = $time;
 
diff --git a/src/Customers/Customer.php b/src/Customers/Customer.php
index 30f48ced..63bcb1a2 100644
--- a/src/Customers/Customer.php
+++ b/src/Customers/Customer.php
@@ -261,7 +261,7 @@ public function getCreatedAt(): ?DateTime
         return $this->createdAt;
     }
 
-    public function setCreatedAt(DateTime $createdAt = null): Customer
+    public function setCreatedAt(?DateTime $createdAt = null): Customer
     {
         $this->createdAt = $createdAt;
 
@@ -273,7 +273,7 @@ public function getUpdatedAt(): ?DateTime
         return $this->updatedAt;
     }
 
-    public function setUpdatedAt(DateTime $updatedAt = null): Customer
+    public function setUpdatedAt(?DateTime $updatedAt = null): Customer
     {
         $this->updatedAt = $updatedAt;
 
@@ -432,9 +432,8 @@ public function setChatHandles(Collection $chats): Customer
 
     /**
      * @param ChatHandle|string $chat
-     * @param string            $type
      */
-    public function addChatHandle($chat, string $type = null): Customer
+    public function addChatHandle($chat, ?string $type = null): Customer
     {
         if (is_string($chat)) {
             $newChatHandle = new ChatHandle();
@@ -480,9 +479,8 @@ public function setEmails(Collection $emails): Customer
 
     /**
      * @param Email|string $email
-     * @param string       $type
      */
-    public function addEmail($email, string $type = null): Customer
+    public function addEmail($email, ?string $type = null): Customer
     {
         if (is_string($email)) {
             $newEmail = new Email();
@@ -528,9 +526,8 @@ public function setPhones(Collection $phones): Customer
 
     /**
      * @param Phone|string $phone
-     * @param string       $type
      */
-    public function addPhone($phone, string $type = null): Customer
+    public function addPhone($phone, ?string $type = null): Customer
     {
         if (is_string($phone)) {
             $newPhone = new Phone();
@@ -566,9 +563,8 @@ public function setSocialProfiles(Collection $socialProfiles): Customer
 
     /**
      * @param SocialProfile|string $profile
-     * @param string               $type
      */
-    public function addSocialProfile($profile, string $type = null): Customer
+    public function addSocialProfile($profile, ?string $type = null): Customer
     {
         if (is_string($profile)) {
             $newProfile = new SocialProfile();
diff --git a/src/Customers/CustomersEndpoint.php b/src/Customers/CustomersEndpoint.php
index 3b91b4fb..4212f490 100644
--- a/src/Customers/CustomersEndpoint.php
+++ b/src/Customers/CustomersEndpoint.php
@@ -34,7 +34,7 @@ public function update(Customer $customer): void
         );
     }
 
-    public function get(int $id, CustomerRequest $customerRequest = null): Customer
+    public function get(int $id, ?CustomerRequest $customerRequest = null): Customer
     {
         $customerResource = $this->restClient->getResource(
             Customer::class,
@@ -51,8 +51,8 @@ public function get(int $id, CustomerRequest $customerRequest = null): Customer
      * @return Customer[]|PagedCollection
      */
     public function list(
-        CustomerFilters $customerFilters = null,
-        CustomerRequest $customerRequest = null
+        ?CustomerFilters $customerFilters = null,
+        ?CustomerRequest $customerRequest = null
     ): PagedCollection {
         $uri = '/v2/customers';
         if ($customerFilters) {
diff --git a/src/Exception/ValidationErrorException.php b/src/Exception/ValidationErrorException.php
index f8dbab78..a82529ba 100644
--- a/src/Exception/ValidationErrorException.php
+++ b/src/Exception/ValidationErrorException.php
@@ -24,7 +24,7 @@ public function __construct(
         VndError $error,
         RequestInterface $request,
         ResponseInterface $response,
-        \Exception $previous = null
+        ?\Exception $previous = null
     ) {
         // Append some details on what steps to take to see the underlying validation problems are.
         $message = $message.' - use getError() to see the underlying validation issues';
diff --git a/src/Http/Authenticator.php b/src/Http/Authenticator.php
index 25c491a4..cbad4db7 100644
--- a/src/Http/Authenticator.php
+++ b/src/Http/Authenticator.php
@@ -49,7 +49,7 @@ class Authenticator
      */
     private $tokenRefreshedCallback;
 
-    public function __construct(Client $client, Auth $auth = null)
+    public function __construct(Client $client, ?Auth $auth = null)
     {
         $this->client = $client;
         $this->auth = $auth ?? new NullCredentials();
diff --git a/src/Http/Hal/VndError.php b/src/Http/Hal/VndError.php
index ca90d71b..2b10457f 100644
--- a/src/Http/Hal/VndError.php
+++ b/src/Http/Hal/VndError.php
@@ -26,7 +26,7 @@ class VndError
      */
     private $errors = [];
 
-    public function __construct(string $message, string $logRef = null, string $path = null)
+    public function __construct(string $message, ?string $logRef = null, ?string $path = null)
     {
         $this->message = $message;
         $this->logRef = $logRef;
diff --git a/src/Http/RestClientBuilder.php b/src/Http/RestClientBuilder.php
index ce3b7059..aa02d104 100644
--- a/src/Http/RestClientBuilder.php
+++ b/src/Http/RestClientBuilder.php
@@ -126,8 +126,8 @@ protected function getRetryDecider(): callable
         return function (
             $retries,
             Request $request,
-            Response $response = null,
-            GuzzleException $exception = null
+            ?Response $response = null,
+            ?GuzzleException $exception = null
         ) {
             // Don't retry unless this is a Connection issue
             if (!$exception instanceof ConnectException) {
diff --git a/src/Mailboxes/Mailbox.php b/src/Mailboxes/Mailbox.php
index e9c97b86..b82a1353 100644
--- a/src/Mailboxes/Mailbox.php
+++ b/src/Mailboxes/Mailbox.php
@@ -99,7 +99,7 @@ public function getCreatedAt()
     /**
      * @return Mailbox
      */
-    public function setCreatedAt(DateTime $createdAt = null): self
+    public function setCreatedAt(?DateTime $createdAt = null): self
     {
         $this->createdAt = $createdAt;
 
@@ -117,7 +117,7 @@ public function getUpdatedAt()
     /**
      * @return Mailbox
      */
-    public function setUpdatedAt(DateTime $updatedAt = null): self
+    public function setUpdatedAt(?DateTime $updatedAt = null): self
     {
         $this->updatedAt = $updatedAt;
 
diff --git a/src/Mailboxes/MailboxesEndpoint.php b/src/Mailboxes/MailboxesEndpoint.php
index dde2a8a9..e3b8d607 100644
--- a/src/Mailboxes/MailboxesEndpoint.php
+++ b/src/Mailboxes/MailboxesEndpoint.php
@@ -15,7 +15,7 @@ class MailboxesEndpoint extends Endpoint
     public const LIST_MAILBOXES_URI = '/v2/mailboxes';
     public const RESOURCE_KEY = 'mailboxes';
 
-    public function get(int $id, MailboxRequest $mailboxRequest = null): Mailbox
+    public function get(int $id, ?MailboxRequest $mailboxRequest = null): Mailbox
     {
         $mailboxResource = $this->restClient->getResource(
             Mailbox::class,
@@ -30,7 +30,7 @@ public function get(int $id, MailboxRequest $mailboxRequest = null): Mailbox
     /**
      * @return Mailbox[]|PagedCollection
      */
-    public function list(MailboxRequest $mailboxRequest = null): PagedCollection
+    public function list(?MailboxRequest $mailboxRequest = null): PagedCollection
     {
         return $this->loadMailboxes(
             self::LIST_MAILBOXES_URI,
diff --git a/src/Teams/Team.php b/src/Teams/Team.php
index ab195ecf..aa30c2f2 100644
--- a/src/Teams/Team.php
+++ b/src/Teams/Team.php
@@ -159,7 +159,7 @@ public function getCreatedAt(): ?DateTime
         return $this->createdAt;
     }
 
-    public function setCreatedAt(DateTime $createdAt = null): Team
+    public function setCreatedAt(?DateTime $createdAt = null): Team
     {
         $this->createdAt = $createdAt;
 
@@ -171,7 +171,7 @@ public function getUpdatedAt(): ?DateTime
         return $this->updatedAt;
     }
 
-    public function setUpdatedAt(DateTime $updatedAt = null): Team
+    public function setUpdatedAt(?DateTime $updatedAt = null): Team
     {
         $this->updatedAt = $updatedAt;
 
diff --git a/src/Users/User.php b/src/Users/User.php
index 0e7973d0..be95b1a3 100644
--- a/src/Users/User.php
+++ b/src/Users/User.php
@@ -141,7 +141,7 @@ public function getCreatedAt(): ?DateTime
         return $this->createdAt;
     }
 
-    public function setCreatedAt(DateTime $createdAt = null): User
+    public function setCreatedAt(?DateTime $createdAt = null): User
     {
         $this->createdAt = $createdAt;
 
@@ -153,7 +153,7 @@ public function getUpdatedAt(): ?DateTime
         return $this->updatedAt;
     }
 
-    public function setUpdatedAt(DateTime $updatedAt = null): User
+    public function setUpdatedAt(?DateTime $updatedAt = null): User
     {
         $this->updatedAt = $updatedAt;
 
diff --git a/src/Users/UsersEndpoint.php b/src/Users/UsersEndpoint.php
index 2ab76aec..12750fa8 100644
--- a/src/Users/UsersEndpoint.php
+++ b/src/Users/UsersEndpoint.php
@@ -34,7 +34,7 @@ public function getAuthenticatedUser(): User
      * @return User[]|PagedCollection
      */
     public function list(
-        UserFilters $userFilters = null
+        ?UserFilters $userFilters = null
     ): PagedCollection {
         $uri = self::LIST_USERS_URI;
         if ($userFilters) {