diff --git a/.travis.yml b/.travis.yml
index 334c65d..94f7e52 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -17,6 +17,7 @@ matrix:
         - php: 8.1
         - php: 8.2
         - php: 8.3
+        - php: 8.4
 
 cache:
   directories:
diff --git a/src/Exchanger.php b/src/Exchanger.php
index aa06e07..42cf6ac 100755
--- a/src/Exchanger.php
+++ b/src/Exchanger.php
@@ -56,7 +56,7 @@ final class Exchanger implements ExchangeRateProviderContract
      * @param CacheInterface|null         $cache
      * @param array                       $options
      */
-    public function __construct(ExchangeRateServiceContract $service, CacheInterface $cache = null, array $options = [])
+    public function __construct(ExchangeRateServiceContract $service, ?CacheInterface $cache = null, array $options = [])
     {
         $this->service = $service;
         $this->cache = $cache;
diff --git a/src/Service/CentralBankOfRepublicTurkey.php b/src/Service/CentralBankOfRepublicTurkey.php
index 1da8371..82125cd 100755
--- a/src/Service/CentralBankOfRepublicTurkey.php
+++ b/src/Service/CentralBankOfRepublicTurkey.php
@@ -70,7 +70,7 @@ public function supportQuery(ExchangeRateQuery $exchangeRateQuery): bool
      *
      * @throws UnsupportedCurrencyPairException
      */
-    private function doCreateRate(ExchangeRateQuery $exchangeQuery, DateTimeInterface $requestedDate = null): ExchangeRate
+    private function doCreateRate(ExchangeRateQuery $exchangeQuery, ?DateTimeInterface $requestedDate = null): ExchangeRate
     {
         $currencyPair = $exchangeQuery->getCurrencyPair();
         $content = $this->request($this->buildUrl($requestedDate));
@@ -97,7 +97,7 @@ private function doCreateRate(ExchangeRateQuery $exchangeQuery, DateTimeInterfac
      *
      * @return string
      */
-    private function buildUrl(DateTimeInterface $requestedDate = null): string
+    private function buildUrl(?DateTimeInterface $requestedDate = null): string
     {
         if (null === $requestedDate) {
             $fileName = 'today';
diff --git a/src/Service/CentralBankOfRepublicUzbekistan.php b/src/Service/CentralBankOfRepublicUzbekistan.php
index 7836ea7..470aef3 100644
--- a/src/Service/CentralBankOfRepublicUzbekistan.php
+++ b/src/Service/CentralBankOfRepublicUzbekistan.php
@@ -77,7 +77,7 @@ protected function getHistoricalExchangeRate(HistoricalExchangeRateQuery $exchan
      * @throws UnsupportedCurrencyPairException
      * @throws Exception
      */
-    private function doCreateRate(ExchangeRateQuery $exchangeQuery, DateTimeInterface $requestedDate = null): ExchangeRate
+    private function doCreateRate(ExchangeRateQuery $exchangeQuery, ?DateTimeInterface $requestedDate = null): ExchangeRate
     {
         $currencyPair = $exchangeQuery->getCurrencyPair();
 
@@ -106,7 +106,7 @@ private function doCreateRate(ExchangeRateQuery $exchangeQuery, DateTimeInterfac
      *
      * @return string
      */
-    private function buildUrl(DateTimeInterface $requestedDate = null): string
+    private function buildUrl(?DateTimeInterface $requestedDate = null): string
     {
         $date = '';
         if (!is_null($requestedDate)) {
diff --git a/src/Service/HttpService.php b/src/Service/HttpService.php
index c24daf0..d6a5d6c 100644
--- a/src/Service/HttpService.php
+++ b/src/Service/HttpService.php
@@ -47,7 +47,7 @@ abstract class HttpService extends Service
      * @param RequestFactoryInterface|null    $requestFactory
      * @param array                           $options
      */
-    public function __construct($httpClient = null, RequestFactoryInterface $requestFactory = null, array $options = [])
+    public function __construct($httpClient = null, ?RequestFactoryInterface $requestFactory = null, array $options = [])
     {
         if (null === $httpClient) {
             $httpClient = HttpClientDiscovery::find();
diff --git a/src/Service/NationalBankOfGeorgia.php b/src/Service/NationalBankOfGeorgia.php
index f4becdf..813920c 100644
--- a/src/Service/NationalBankOfGeorgia.php
+++ b/src/Service/NationalBankOfGeorgia.php
@@ -77,7 +77,7 @@ protected function getHistoricalExchangeRate(HistoricalExchangeRateQuery $exchan
      * @throws UnsupportedCurrencyPairException
      * @throws Exception
      */
-    private function doCreateRate(ExchangeRateQuery $exchangeQuery, DateTimeInterface $requestedDate = null): ExchangeRate
+    private function doCreateRate(ExchangeRateQuery $exchangeQuery, ?DateTimeInterface $requestedDate = null): ExchangeRate
     {
         $currencyPair = $exchangeQuery->getCurrencyPair();
 
@@ -106,7 +106,7 @@ private function doCreateRate(ExchangeRateQuery $exchangeQuery, DateTimeInterfac
      *
      * @return string
      */
-    private function buildUrl(DateTimeInterface $requestedDate = null): string
+    private function buildUrl(?DateTimeInterface $requestedDate = null): string
     {
         $date = '';
         if (!is_null($requestedDate)) {
diff --git a/src/Service/NationalBankOfRepublicBelarus.php b/src/Service/NationalBankOfRepublicBelarus.php
index 6b16932..d171205 100644
--- a/src/Service/NationalBankOfRepublicBelarus.php
+++ b/src/Service/NationalBankOfRepublicBelarus.php
@@ -81,7 +81,7 @@ public function supportQuery(ExchangeRateQuery $exchangeQuery, bool $ignoreSuppo
      *
      * @return int|false
      */
-    private static function detectPeriodicity(string $baseCurrency, \DateTimeInterface $date = null)
+    private static function detectPeriodicity(string $baseCurrency, ?\DateTimeInterface $date = null)
     {
         return array_reduce(
 
@@ -115,7 +115,7 @@ static function ($periodicity, $entry) use ($date) {
      *
      * @return bool
      */
-    private static function supportQuoteCurrency(string $quoteCurrency, \DateTimeInterface $date = null): bool
+    private static function supportQuoteCurrency(string $quoteCurrency, ?\DateTimeInterface $date = null): bool
     {
         if ($date) {
             $date = $date->format('Y-m-d');
@@ -167,7 +167,7 @@ public function getName(): string
      * @throws UnsupportedDateException
      * @throws UnsupportedExchangeQueryException
      */
-    private function doCreateRate(ExchangeRateQuery $exchangeQuery, \DateTimeInterface $requestedDate = null): ExchangeRate
+    private function doCreateRate(ExchangeRateQuery $exchangeQuery, ?\DateTimeInterface $requestedDate = null): ExchangeRate
     {
         $currencyPair = $exchangeQuery->getCurrencyPair();
         $baseCurrency = $currencyPair->getBaseCurrency();
@@ -224,7 +224,7 @@ private function doCreateRate(ExchangeRateQuery $exchangeQuery, \DateTimeInterfa
      *
      * @return string
      */
-    private function buildUrl(string $baseCurrency, \DateTimeInterface $requestedDate = null): string
+    private function buildUrl(string $baseCurrency, ?\DateTimeInterface $requestedDate = null): string
     {
         $data = isset($requestedDate) ? ['ondate' => $requestedDate->format('Y-m-d')] : [];
         $data += ['periodicity' => (int) self::detectPeriodicity($baseCurrency, $requestedDate)];