diff --git a/src/Contact.php b/src/Contact.php
index f554830..0972969 100644
--- a/src/Contact.php
+++ b/src/Contact.php
@@ -70,7 +70,7 @@ class Contact extends AbstractResource
      * @param  ClientInterface|null $client
      * @return Contact
      */
-    public static function merge($into, $from, ClientInterface $client = null)
+    public static function merge($into, $from, ?ClientInterface $client = null)
     {
         $result = (new static([], $client))
             ->getClient()
diff --git a/src/Customer.php b/src/Customer.php
index 174aed1..4ca31b6 100644
--- a/src/Customer.php
+++ b/src/Customer.php
@@ -112,7 +112,7 @@ public function customAttributes()
      * @param  ClientInterface|null $client
      * @return Customer|null
      */
-    public static function findByExternalId($externalId, ClientInterface $client = null)
+    public static function findByExternalId($externalId, ?ClientInterface $client = null)
     {
         if (gettype($externalId) == 'string') {
             $externalId = ['external_id' => $externalId];
@@ -136,7 +136,7 @@ public static function findByExternalId($externalId, ClientInterface $client = n
      * @param  ClientInterface|null $client
      * @return Collection|static
      */
-    public static function search($email, ClientInterface $client = null)
+    public static function search($email, ?ClientInterface $client = null)
     {
         $response = (new static([], $client))
             ->getClient()
@@ -154,7 +154,7 @@ public static function search($email, ClientInterface $client = null)
      * @param  ClientInterface|null $client
      * @return bool
      */
-    public static function merge($from, $into, ClientInterface $client = null)
+    public static function merge($from, $into, ?ClientInterface $client = null)
     {
         (new static([], $client))
             ->getClient()
@@ -180,7 +180,7 @@ public static function merge($from, $into, ClientInterface $client = null)
      * @param  ClientInterface|null $client
      * @return bool
      */
-    public static function unmerge($customer_uuid, $external_id, $data_source_uuid, array $move_to_new_customer = [], ClientInterface $client = null)
+    public static function unmerge($customer_uuid, $external_id, $data_source_uuid, array $move_to_new_customer = [], ?ClientInterface $client = null)
     {
         (new static([], $client))
             ->getClient()
@@ -206,7 +206,7 @@ public static function unmerge($customer_uuid, $external_id, $data_source_uuid,
      * @param  ClientInterface|null $client
      * @return bool
      */
-    public static function connectSubscriptions($customerUUID, array $data = [], ClientInterface $client = null)
+    public static function connectSubscriptions($customerUUID, array $data = [], ?ClientInterface $client = null)
     {
         (new static([], $client))
             ->getClient()
diff --git a/src/CustomerInvoices.php b/src/CustomerInvoices.php
index c3ca903..fe9fc6a 100644
--- a/src/CustomerInvoices.php
+++ b/src/CustomerInvoices.php
@@ -24,7 +24,7 @@ class CustomerInvoices extends AbstractResource
 
     public $customer_uuid;
 
-    public function __construct(array $attr = [], ClientInterface $client = null)
+    public function __construct(array $attr = [], ?ClientInterface $client = null)
     {
         parent::__construct($attr, $client);
 
@@ -34,7 +34,7 @@ public function __construct(array $attr = [], ClientInterface $client = null)
         }
     }
 
-    public static function destroyAll($dataSourceUUID, $customerUUID, ClientInterface $client = null)
+    public static function destroyAll($dataSourceUUID, $customerUUID, ?ClientInterface $client = null)
     {
         (new static([], $client))
             ->getClient()
diff --git a/src/Customers.php b/src/Customers.php
index 7e9609b..3e08057 100644
--- a/src/Customers.php
+++ b/src/Customers.php
@@ -46,7 +46,7 @@ protected static function getEntryClass()
      * @param array                $attr
      * @param ClientInterface|null $client
      */
-    public function __construct(array $attr = [], ClientInterface $client = null)
+    public function __construct(array $attr = [], ?ClientInterface $client = null)
     {
         parent::__construct($attr, $client);
 
@@ -61,7 +61,7 @@ public function __construct(array $attr = [], ClientInterface $client = null)
      * @return     Collection|static
      * @deprecated Use Customer.
      */
-    public static function search($email, ClientInterface $client = null)
+    public static function search($email, ?ClientInterface $client = null)
     {
         return Customer::search($email, $client);
     }
diff --git a/src/Exceptions/ChartMogulException.php b/src/Exceptions/ChartMogulException.php
index c378abd..30fd3c9 100644
--- a/src/Exceptions/ChartMogulException.php
+++ b/src/Exceptions/ChartMogulException.php
@@ -28,7 +28,7 @@ class ChartMogulException extends \RuntimeException implements ResponseException
      * @param ResponseInterface|null $response ResponseInterface object
      * @param \Exception|null        $previous
      */
-    public function __construct($message, ResponseInterface $response = null, \Exception $previous = null)
+    public function __construct($message, ?ResponseInterface $response = null, ?\Exception $previous = null)
     {
         if ($response) {
             $response->getBody()->rewind();
diff --git a/src/Http/Client.php b/src/Http/Client.php
index 7567202..1348a43 100644
--- a/src/Http/Client.php
+++ b/src/Http/Client.php
@@ -49,9 +49,9 @@ class Client implements ClientInterface
      * @codeCoverageIgnore
      */
     public function __construct(
-        Configuration $config = null,
-        HttpClient $client = null,
-        RequestFactoryInterface $requestFactory = null
+        ?Configuration $config = null,
+        ?HttpClient $client = null,
+        ?RequestFactoryInterface $requestFactory = null
     ) {
         $this->config = $config ?: Configuration::getDefaultConfiguration();
         $this->client = $client ?: Psr18ClientDiscovery::find();
diff --git a/src/Http/Retry.php b/src/Http/Retry.php
index f4cd24b..aaeb13d 100644
--- a/src/Http/Retry.php
+++ b/src/Http/Retry.php
@@ -25,7 +25,7 @@ private function retryHTTPStatus($status)
         return $status == 429 || ($status >= 500 && $status < 600);
     }
 
-    protected function shouldRetry($attempt, $maxAttempts, ResponseInterface $response = null, \Exception $ex = null)
+    protected function shouldRetry($attempt, $maxAttempts, ?ResponseInterface $response = null, ?\Exception $ex = null)
     {
         if ($attempt >= $maxAttempts && !is_null($ex)) {
             throw  $ex;
diff --git a/src/Invoice.php b/src/Invoice.php
index ed2d03f..43b34f6 100644
--- a/src/Invoice.php
+++ b/src/Invoice.php
@@ -51,7 +51,7 @@ class Invoice extends AbstractResource
     public $line_items = [];
     public $transactions = [];
 
-    public function __construct(array $attr = [], ClientInterface $client = null)
+    public function __construct(array $attr = [], ?ClientInterface $client = null)
     {
         parent::__construct($attr, $client);
 
diff --git a/src/Metrics.php b/src/Metrics.php
index ead0ecc..5de4d44 100644
--- a/src/Metrics.php
+++ b/src/Metrics.php
@@ -17,7 +17,7 @@ class Metrics
      * @param  ClientInterface|null $client
      * @return NSMetrics\AllKeyMetrics
      */
-    public static function all(array $options = [], ClientInterface $client = null)
+    public static function all(array $options = [], ?ClientInterface $client = null)
     {
         return NSMetrics\AllKeyMetrics::all($options, $client);
     }
@@ -26,7 +26,7 @@ public static function all(array $options = [], ClientInterface $client = null)
      *
      * @inheritDoc
      */
-    public static function arpa(array $options = [], ClientInterface $client = null)
+    public static function arpa(array $options = [], ?ClientInterface $client = null)
     {
         return NSMetrics\ARPAs::all($options, $client);
     }
@@ -36,7 +36,7 @@ public static function arpa(array $options = [], ClientInterface $client = null)
      *
      * @inheritDoc
      */
-    public static function arr(array $options = [], ClientInterface $client = null)
+    public static function arr(array $options = [], ?ClientInterface $client = null)
     {
         return NSMetrics\ARRs::all($options, $client);
     }
@@ -46,7 +46,7 @@ public static function arr(array $options = [], ClientInterface $client = null)
      *
      * @inheritDoc
      */
-    public static function asp(array $options = [], ClientInterface $client = null)
+    public static function asp(array $options = [], ?ClientInterface $client = null)
     {
         return NSMetrics\ASPs::all($options, $client);
     }
@@ -56,7 +56,7 @@ public static function asp(array $options = [], ClientInterface $client = null)
      *
      * @inheritDoc
      */
-    public static function customerChurnRate(array $options = [], ClientInterface $client = null)
+    public static function customerChurnRate(array $options = [], ?ClientInterface $client = null)
     {
         return NSMetrics\CustomerChurnRates::all($options, $client);
     }
@@ -66,7 +66,7 @@ public static function customerChurnRate(array $options = [], ClientInterface $c
      *
      * @inheritDoc
      */
-    public static function customerCount(array $options = [], ClientInterface $client = null)
+    public static function customerCount(array $options = [], ?ClientInterface $client = null)
     {
         return NSMetrics\CustomerCounts::all($options, $client);
     }
@@ -76,7 +76,7 @@ public static function customerCount(array $options = [], ClientInterface $clien
      *
      * @inheritDoc
      */
-    public static function mrr(array $options = [], ClientInterface $client = null)
+    public static function mrr(array $options = [], ?ClientInterface $client = null)
     {
         return NSMetrics\MRRs::all($options, $client);
     }
@@ -86,7 +86,7 @@ public static function mrr(array $options = [], ClientInterface $client = null)
      *
      * @inheritDoc
      */
-    public static function ltv(array $options = [], ClientInterface $client = null)
+    public static function ltv(array $options = [], ?ClientInterface $client = null)
     {
         return NSMetrics\LTVs::all($options, $client);
     }
@@ -96,7 +96,7 @@ public static function ltv(array $options = [], ClientInterface $client = null)
      *
      * @inheritDoc
      */
-    public static function mrrChurnRate(array $options = [], ClientInterface $client = null)
+    public static function mrrChurnRate(array $options = [], ?ClientInterface $client = null)
     {
         return NSMetrics\MRRChurnRates::all($options, $client);
     }
diff --git a/src/Metrics/AbstractMetric.php b/src/Metrics/AbstractMetric.php
index 2227b85..920ae65 100644
--- a/src/Metrics/AbstractMetric.php
+++ b/src/Metrics/AbstractMetric.php
@@ -24,7 +24,7 @@ protected static function getEntryClass()
         return static::ENTRY_CLASS;
     }
 
-    public function __construct(array $attr = [], ClientInterface $client = null)
+    public function __construct(array $attr = [], ?ClientInterface $client = null)
     {
         parent::__construct($attr, $client);
 
diff --git a/src/Metrics/Activities.php b/src/Metrics/Activities.php
index 9312394..1bf0377 100644
--- a/src/Metrics/Activities.php
+++ b/src/Metrics/Activities.php
@@ -29,7 +29,7 @@ protected static function getEntryClass()
         return static::ENTRY_CLASS;
     }
 
-    public function __construct(array $attr = [], ClientInterface $client = null)
+    public function __construct(array $attr = [], ?ClientInterface $client = null)
     {
         parent::__construct($attr, $client);
 
diff --git a/src/Metrics/Activity.php b/src/Metrics/Activity.php
index 9368a55..8d007ce 100644
--- a/src/Metrics/Activity.php
+++ b/src/Metrics/Activity.php
@@ -45,7 +45,7 @@ class Activity extends AbstractModel
      * @param  ClientInterface|null $client
      * @return Activities
      */
-    public static function all(array $options = [], ClientInterface $client = null)
+    public static function all(array $options = [], ?ClientInterface $client = null)
     {
         return Activities::all($options, $client);
     }
diff --git a/src/Metrics/AllKeyMetrics.php b/src/Metrics/AllKeyMetrics.php
index 6b27262..efe382b 100644
--- a/src/Metrics/AllKeyMetrics.php
+++ b/src/Metrics/AllKeyMetrics.php
@@ -29,7 +29,7 @@ protected static function getEntryClass()
         return static::ENTRY_CLASS;
     }
 
-    public function __construct(array $attr = [], ClientInterface $client = null)
+    public function __construct(array $attr = [], ?ClientInterface $client = null)
     {
         parent::__construct($attr, $client);
 
diff --git a/src/Metrics/Customers/Activities.php b/src/Metrics/Customers/Activities.php
index e022511..5d7f727 100644
--- a/src/Metrics/Customers/Activities.php
+++ b/src/Metrics/Customers/Activities.php
@@ -29,7 +29,7 @@ protected static function getEntryClass()
         return static::ENTRY_CLASS;
     }
 
-    public function __construct(array $attr = [], ClientInterface $client = null)
+    public function __construct(array $attr = [], ?ClientInterface $client = null)
     {
         parent::__construct($attr, $client);
 
diff --git a/src/Metrics/Customers/Activity.php b/src/Metrics/Customers/Activity.php
index 15b7991..2be81d6 100644
--- a/src/Metrics/Customers/Activity.php
+++ b/src/Metrics/Customers/Activity.php
@@ -37,7 +37,7 @@ class Activity extends AbstractModel
      * @param  ClientInterface|null $client
      * @return Activities
      */
-    public static function all(array $options = [], ClientInterface $client = null)
+    public static function all(array $options = [], ?ClientInterface $client = null)
     {
         return Activities::all($options, $client);
     }
diff --git a/src/Metrics/Customers/Subscription.php b/src/Metrics/Customers/Subscription.php
index 1d1380d..affe3e7 100644
--- a/src/Metrics/Customers/Subscription.php
+++ b/src/Metrics/Customers/Subscription.php
@@ -42,7 +42,7 @@ class Subscription extends AbstractModel
      * @param  ClientInterface|null $client
      * @return Subscriptions
      */
-    public static function all(array $options = [], ClientInterface $client = null)
+    public static function all(array $options = [], ?ClientInterface $client = null)
     {
         return Subscriptions::all($options, $client);
     }
diff --git a/src/Metrics/Customers/Subscriptions.php b/src/Metrics/Customers/Subscriptions.php
index b046053..458a9c8 100644
--- a/src/Metrics/Customers/Subscriptions.php
+++ b/src/Metrics/Customers/Subscriptions.php
@@ -29,7 +29,7 @@ protected static function getEntryClass()
         return static::ENTRY_CLASS;
     }
 
-    public function __construct(array $attr = [], ClientInterface $client = null)
+    public function __construct(array $attr = [], ?ClientInterface $client = null)
     {
         parent::__construct($attr, $client);
 
diff --git a/src/Ping.php b/src/Ping.php
index b5696a9..fcaffe5 100644
--- a/src/Ping.php
+++ b/src/Ping.php
@@ -25,7 +25,7 @@ class Ping extends AbstractResource
 
     public $data;
 
-    public static function ping(ClientInterface $client = null)
+    public static function ping(?ClientInterface $client = null)
     {
         return Ping::all([], $client);
     }
diff --git a/src/Resource/AbstractResource.php b/src/Resource/AbstractResource.php
index be9ea71..a07ce59 100644
--- a/src/Resource/AbstractResource.php
+++ b/src/Resource/AbstractResource.php
@@ -34,7 +34,7 @@ abstract class AbstractResource extends AbstractModel
      * @param              ClientInterface|null $client
      * @return             self
      */
-    public function __construct(array $attr = [], ClientInterface $client = null)
+    public function __construct(array $attr = [], ?ClientInterface $client = null)
     {
         parent::__construct($attr);
 
@@ -69,7 +69,7 @@ public function setClient(ClientInterface $client)
      * @param  ClientInterface|null $client
      * @return Collection|static
      */
-    public static function fromArray(array $data, ClientInterface $client = null)
+    public static function fromArray(array $data, ?ClientInterface $client = null)
     {
         if (isset($data[static::ROOT_KEY])) {
             if (static::ROOT_KEY != "subscription_events") {
diff --git a/src/Service/AllTrait.php b/src/Service/AllTrait.php
index 5ae5b04..80c7ae9 100644
--- a/src/Service/AllTrait.php
+++ b/src/Service/AllTrait.php
@@ -17,7 +17,7 @@ trait AllTrait
      * @param  ClientInterface|null $client 0
      * @return Collection|self[]|self
      */
-    public static function all(array $data = [], ClientInterface $client = null)
+    public static function all(array $data = [], ?ClientInterface $client = null)
     {
         return (new RequestService($client))
             ->setResourceClass(static::class)
diff --git a/src/Service/CreateTrait.php b/src/Service/CreateTrait.php
index eafc12d..a0806e1 100644
--- a/src/Service/CreateTrait.php
+++ b/src/Service/CreateTrait.php
@@ -16,7 +16,7 @@ trait CreateTrait
      * @param  ClientInterface|null $client
      * @return self
      */
-    public static function create(array $data = [], ClientInterface $client = null)
+    public static function create(array $data = [], ?ClientInterface $client = null)
     {
         return (new RequestService($client))
             ->setResourceClass(static::class)
diff --git a/src/Service/DestroyWithParamsTrait.php b/src/Service/DestroyWithParamsTrait.php
index d6f697c..736d716 100644
--- a/src/Service/DestroyWithParamsTrait.php
+++ b/src/Service/DestroyWithParamsTrait.php
@@ -14,7 +14,7 @@ trait DestroyWithParamsTrait
      *
      * @return boolean
      */
-    public static function destroyWithParams(array $params = [], ClientInterface $client = null)
+    public static function destroyWithParams(array $params = [], ?ClientInterface $client = null)
     {
         return (new RequestService($client))
             ->setResourceClass(static::class)
diff --git a/src/Service/FromArrayTrait.php b/src/Service/FromArrayTrait.php
index dd2dfd9..b035107 100644
--- a/src/Service/FromArrayTrait.php
+++ b/src/Service/FromArrayTrait.php
@@ -17,7 +17,7 @@ trait FromArrayTrait
      * @param  ClientInterface|null $client
      * @return CollectionWithCursor|static
      */
-    public static function fromArray(array $data, ClientInterface $client = null)
+    public static function fromArray(array $data, ?ClientInterface $client = null)
     {
         if (isset($data[static::ROOT_KEY])) {
             $array = new CollectionWithCursor(
diff --git a/src/Service/GetTrait.php b/src/Service/GetTrait.php
index 217732d..e39e25e 100644
--- a/src/Service/GetTrait.php
+++ b/src/Service/GetTrait.php
@@ -14,14 +14,14 @@ trait GetTrait
      *
      * @return resource
      */
-    public static function retrieve($uuid, ClientInterface $client = null)
+    public static function retrieve($uuid, ?ClientInterface $client = null)
     {
         return (new RequestService($client))
             ->setResourceClass(static::class)
             ->get($uuid);
     }
 
-    public static function get($uuid, ClientInterface $client = null)
+    public static function get($uuid, ?ClientInterface $client = null)
     {
         return static::retrieve($uuid, $client);
     }
diff --git a/src/Service/RequestService.php b/src/Service/RequestService.php
index 1a2c414..9160896 100644
--- a/src/Service/RequestService.php
+++ b/src/Service/RequestService.php
@@ -15,7 +15,7 @@ class RequestService
     private $resourcePath;
     private $client;
 
-    public function __construct(ClientInterface $client = null)
+    public function __construct(?ClientInterface $client = null)
     {
         if (is_null($client)) {
             $client = new Client();
diff --git a/src/Service/ShowTrait.php b/src/Service/ShowTrait.php
index e392990..250dac7 100644
--- a/src/Service/ShowTrait.php
+++ b/src/Service/ShowTrait.php
@@ -15,7 +15,7 @@ trait ShowTrait
      * @return resource
      */
 
-    public static function retrieve(ClientInterface $client = null)
+    public static function retrieve(?ClientInterface $client = null)
     {
         return (new RequestService($client))
             ->setResourceClass(static::class)
diff --git a/src/Service/UpdateTrait.php b/src/Service/UpdateTrait.php
index 869e0c5..4cf226c 100644
--- a/src/Service/UpdateTrait.php
+++ b/src/Service/UpdateTrait.php
@@ -16,7 +16,7 @@ trait UpdateTrait
      * @param  ClientInterface|null $client
      * @return self
      */
-    public static function update(array $id = [], array $data = [], ClientInterface $client = null)
+    public static function update(array $id = [], array $data = [], ?ClientInterface $client = null)
     {
         return (new RequestService($client))
             ->setResourceClass(static::class)
diff --git a/src/Service/UpdateWithParamsTrait.php b/src/Service/UpdateWithParamsTrait.php
index 5d898aa..0197828 100644
--- a/src/Service/UpdateWithParamsTrait.php
+++ b/src/Service/UpdateWithParamsTrait.php
@@ -16,7 +16,7 @@ trait UpdateWithParamsTrait
      * @param  ClientInterface|null $client
      * @return self
      */
-    public static function updateWithParams(array $params = [], ClientInterface $client = null)
+    public static function updateWithParams(array $params = [], ?ClientInterface $client = null)
     {
         return (new RequestService($client))
             ->setResourceClass(static::class)
diff --git a/src/Subscription.php b/src/Subscription.php
index 1d04b2b..5f9042d 100644
--- a/src/Subscription.php
+++ b/src/Subscription.php
@@ -93,7 +93,7 @@ public function setCancellationDates($cancellationDates)
     /**
      * @inheritDoc
      */
-    public static function fromArray(array $data, ClientInterface $client = null)
+    public static function fromArray(array $data, ?ClientInterface $client = null)
     {
         $result = parent::fromArray($data, $client);
         if (isset($data["customer_uuid"]) && $result instanceof Collection) {