diff --git a/src/CurrencyService.php b/src/CurrencyService.php
index 5638fd8..a36feef 100644
--- a/src/CurrencyService.php
+++ b/src/CurrencyService.php
@@ -22,7 +22,7 @@ class CurrencyService implements CurrencyServiceInterface, ExchangeServiceInterf
     protected $exchangeArgs = [];
 
 
-    public function __construct(ExchangeRateProviderInterface $exchange = null)
+    public function __construct(?ExchangeRateProviderInterface $exchange = null)
     {
         $exchange !== null && $this->setExchangeRateProvider($exchange);
     }
@@ -262,7 +262,7 @@ public function getExchangeRateProvider(): ExchangeRateProviderInterface
     }
 
 
-    public function setExchangeRateProvider(ExchangeRateProviderInterface $exchange = null): self
+    public function setExchangeRateProvider(?ExchangeRateProviderInterface $exchange = null): self
     {
         $this->exchange = $exchange;
         return $this;
diff --git a/src/Exceptions/MathException.php b/src/Exceptions/MathException.php
index ca835d3..e3382b4 100644
--- a/src/Exceptions/MathException.php
+++ b/src/Exceptions/MathException.php
@@ -12,7 +12,7 @@
  */
 class MathException extends RuntimeException
 {
-    public function __construct($message = null, $code = null, Throwable $previous = null)
+    public function __construct($message = null, $code = null, ?Throwable $previous = null)
     {
         parent::__construct($message ?? '', $code ?? 0, $previous);
     }
diff --git a/src/Exceptions/SetupException.php b/src/Exceptions/SetupException.php
index 8a7ed3a..7957152 100644
--- a/src/Exceptions/SetupException.php
+++ b/src/Exceptions/SetupException.php
@@ -12,7 +12,7 @@
  */
 class SetupException extends LogicException
 {
-    public function __construct($message = null, $code = null, Throwable $previous = null)
+    public function __construct($message = null, $code = null, ?Throwable $previous = null)
     {
         parent::__construct($message ?? '', $code ?? 0, $previous);
     }
diff --git a/src/StaticExchange.php b/src/StaticExchange.php
index 6a78c36..0c351f6 100644
--- a/src/StaticExchange.php
+++ b/src/StaticExchange.php
@@ -71,7 +71,7 @@ public function __construct($referenceCurrency, array $exchangeRates, string $ex
      * @param CurrencyInterface $from
      * @return float|int
      */
-    public function getExchangeRate(CurrencyInterface $target, CurrencyInterface $from = null)
+    public function getExchangeRate(CurrencyInterface $target, ?CurrencyInterface $from = null)
     {
         $getter = function (CurrencyInterface $target) {
             [$rate, $amount, $type] = $this->getRawExchangeRateSetup($target->code());