diff --git a/src/Exceptions/AmountExceedsMolliePaymentMethodLimit.php b/src/Exceptions/AmountExceedsMolliePaymentMethodLimit.php
index ded5205d..36470ec7 100644
--- a/src/Exceptions/AmountExceedsMolliePaymentMethodLimit.php
+++ b/src/Exceptions/AmountExceedsMolliePaymentMethodLimit.php
@@ -14,7 +14,7 @@ class AmountExceedsMolliePaymentMethodLimit extends Exception
      * @param  int  $code
      * @param  Throwable|null  $previous
      */
-    public function __construct(string $message = 'The order total is greater than the total accepted by Mollie', int $code = 500, Throwable $previous = null)
+    public function __construct(string $message = 'The order total is greater than the total accepted by Mollie', int $code = 500, ?Throwable $previous = null)
     {
         parent::__construct($message, $code, $previous);
     }
diff --git a/src/Exceptions/CouponException.php b/src/Exceptions/CouponException.php
index 78c2a3b2..68371383 100644
--- a/src/Exceptions/CouponException.php
+++ b/src/Exceptions/CouponException.php
@@ -14,7 +14,7 @@ class CouponException extends Exception
      * @param  int  $code
      * @param  \Throwable|null  $previous
      */
-    public function __construct(string $message, int $code = 404, Throwable $previous = null)
+    public function __construct(string $message, int $code = 404, ?Throwable $previous = null)
     {
         parent::__construct($message, $code, $previous);
     }
diff --git a/src/Exceptions/CouponNotFoundException.php b/src/Exceptions/CouponNotFoundException.php
index 1a477c95..04695da1 100644
--- a/src/Exceptions/CouponNotFoundException.php
+++ b/src/Exceptions/CouponNotFoundException.php
@@ -13,7 +13,7 @@ class CouponNotFoundException extends CouponException
      * @param  int  $code
      * @param  Throwable|null  $previous
      */
-    public function __construct(string $message = 'Coupon not found', int $code = 404, Throwable $previous = null)
+    public function __construct(string $message = 'Coupon not found', int $code = 404, ?Throwable $previous = null)
     {
         parent::__construct($message, $code, $previous);
     }
diff --git a/src/Exceptions/CurrencyMismatchException.php b/src/Exceptions/CurrencyMismatchException.php
index 3df8fc19..ec53038f 100644
--- a/src/Exceptions/CurrencyMismatchException.php
+++ b/src/Exceptions/CurrencyMismatchException.php
@@ -14,7 +14,7 @@ class CurrencyMismatchException extends Exception
      * @param  int  $code
      * @param  Throwable|null  $previous
      */
-    public function __construct(string $message = 'Currencies do not match', int $code = 404, Throwable $previous = null)
+    public function __construct(string $message = 'Currencies do not match', int $code = 404, ?Throwable $previous = null)
     {
         parent::__construct($message, $code, $previous);
     }
diff --git a/src/Exceptions/InvalidMandateException.php b/src/Exceptions/InvalidMandateException.php
index b4bb2cc9..b6206d90 100644
--- a/src/Exceptions/InvalidMandateException.php
+++ b/src/Exceptions/InvalidMandateException.php
@@ -14,7 +14,7 @@ class InvalidMandateException extends Exception
      * @param  int  $code
      * @param  Throwable|null  $previous
      */
-    public function __construct(string $message = 'Invalid customer mandate', int $code = 404, Throwable $previous = null)
+    public function __construct(string $message = 'Invalid customer mandate', int $code = 404, ?Throwable $previous = null)
     {
         parent::__construct($message, $code, $previous);
     }
diff --git a/src/Exceptions/MandateIsNotYetFinalizedException.php b/src/Exceptions/MandateIsNotYetFinalizedException.php
index b921a2d7..e0f54b04 100644
--- a/src/Exceptions/MandateIsNotYetFinalizedException.php
+++ b/src/Exceptions/MandateIsNotYetFinalizedException.php
@@ -14,7 +14,7 @@ class MandateIsNotYetFinalizedException extends Exception
      * @param  int  $code
      * @param  Throwable|null  $previous
      */
-    public function __construct(string $message = 'The customer mandate is still pending', int $code = 404, Throwable $previous = null)
+    public function __construct(string $message = 'The customer mandate is still pending', int $code = 404, ?Throwable $previous = null)
     {
         parent::__construct($message, $code, $previous);
     }
diff --git a/src/Exceptions/OrderRetryRequiresStatusFailedException.php b/src/Exceptions/OrderRetryRequiresStatusFailedException.php
index 0a94e53a..63e378f7 100644
--- a/src/Exceptions/OrderRetryRequiresStatusFailedException.php
+++ b/src/Exceptions/OrderRetryRequiresStatusFailedException.php
@@ -14,7 +14,7 @@ class OrderRetryRequiresStatusFailedException extends Exception
      * @param  int  $code
      * @param  Throwable|null  $previous
      */
-    public function __construct(string $message = 'The order status is not failed', int $code = 500, Throwable $previous = null)
+    public function __construct(string $message = 'The order status is not failed', int $code = 500, ?Throwable $previous = null)
     {
         parent::__construct($message, $code, $previous);
     }
diff --git a/src/Exceptions/PlanNotFoundException.php b/src/Exceptions/PlanNotFoundException.php
index 4afb1ec1..d6d954f7 100644
--- a/src/Exceptions/PlanNotFoundException.php
+++ b/src/Exceptions/PlanNotFoundException.php
@@ -14,7 +14,7 @@ class PlanNotFoundException extends Exception
      * @param  int  $code
      * @param  Throwable|null  $previous
      */
-    public function __construct(string $message = 'Plan not found', int $code = 404, Throwable $previous = null)
+    public function __construct(string $message = 'Plan not found', int $code = 404, ?Throwable $previous = null)
     {
         parent::__construct($message, $code, $previous);
     }
diff --git a/src/Order/Invoice.php b/src/Order/Invoice.php
index bfc4059b..00048c89 100644
--- a/src/Order/Invoice.php
+++ b/src/Order/Invoice.php
@@ -461,7 +461,7 @@ public function view(array $data = [], string $view = self::DEFAULT_VIEW)
      * @param  \Dompdf\Options  $options
      * @return string
      */
-    public function pdf(array $data = [], string $view = self::DEFAULT_VIEW, Options $options = null)
+    public function pdf(array $data = [], string $view = self::DEFAULT_VIEW, ?Options $options = null)
     {
         if (!defined('DOMPDF_ENABLE_AUTOLOAD')) {
             define('DOMPDF_ENABLE_AUTOLOAD', false);
@@ -482,7 +482,7 @@ public function pdf(array $data = [], string $view = self::DEFAULT_VIEW, Options
      * @param  \Dompdf\Options  $options
      * @return \Symfony\Component\HttpFoundation\Response
      */
-    public function download(array $data = [], string $view = self::DEFAULT_VIEW, Options $options = null)
+    public function download(array $data = [], string $view = self::DEFAULT_VIEW, ?Options $options = null)
     {
         $filename = implode('_', [
             $this->id,
diff --git a/src/Plan/AdvancedIntervalGenerator.php b/src/Plan/AdvancedIntervalGenerator.php
index 0793eb1c..228214cc 100644
--- a/src/Plan/AdvancedIntervalGenerator.php
+++ b/src/Plan/AdvancedIntervalGenerator.php
@@ -26,7 +26,7 @@ public function __construct(array $configuration)
      * @param  \Laravel\Cashier\Subscription|null  $subscription
      * @return \Carbon\Carbon|\Carbon\Traits\Modifiers
      */
-    public function getEndOfNextSubscriptionCycle(Subscription $subscription = null)
+    public function getEndOfNextSubscriptionCycle(?Subscription $subscription = null)
     {
         $cycle_ends_at = $subscription->cycle_ends_at ?? now();
         $subscription_date = $this->startOfTheSubscription($subscription);
diff --git a/src/Plan/BaseIntervalGenerator.php b/src/Plan/BaseIntervalGenerator.php
index 1d4816d2..cc1000b5 100644
--- a/src/Plan/BaseIntervalGenerator.php
+++ b/src/Plan/BaseIntervalGenerator.php
@@ -11,7 +11,7 @@ class BaseIntervalGenerator
      * @param  \Laravel\Cashier\Subscription|null  $subscription
      * @return \Carbon\Carbon
      */
-    protected function startOfTheSubscription(Subscription $subscription = null)
+    protected function startOfTheSubscription(?Subscription $subscription = null)
     {
         if (isset($subscription->trial_ends_at) && ! is_null($subscription->trial_ends_at)) {
             return $subscription->trial_ends_at;
diff --git a/src/Plan/Contracts/IntervalGeneratorContract.php b/src/Plan/Contracts/IntervalGeneratorContract.php
index cb04323c..5b4b6d0b 100644
--- a/src/Plan/Contracts/IntervalGeneratorContract.php
+++ b/src/Plan/Contracts/IntervalGeneratorContract.php
@@ -10,5 +10,5 @@ interface IntervalGeneratorContract
      * @param  \Laravel\Cashier\Subscription  $subscription
      * @return \Carbon\Carbon
      */
-    public function getEndOfNextSubscriptionCycle(Subscription $subscription = null);
+    public function getEndOfNextSubscriptionCycle(?Subscription $subscription = null);
 }
diff --git a/src/Plan/DefaultIntervalGenerator.php b/src/Plan/DefaultIntervalGenerator.php
index 37ed1d2b..4a30daf7 100644
--- a/src/Plan/DefaultIntervalGenerator.php
+++ b/src/Plan/DefaultIntervalGenerator.php
@@ -23,7 +23,7 @@ public function __construct(string $interval)
      * @param  \Laravel\Cashier\Subscription|null  $subscription
      * @return \Carbon\Carbon|\Carbon\Traits\Modifiers
      */
-    public function getEndOfNextSubscriptionCycle(Subscription $subscription = null)
+    public function getEndOfNextSubscriptionCycle(?Subscription $subscription = null)
     {
         $cycle_ends_at = $subscription->cycle_ends_at ?? now();
         $subscription_date = $this->startOfTheSubscription($subscription);