diff --git a/.php-cs-fixer.dist.php b/.php-cs-fixer.dist.php
index fb636632..93ff7a4c 100644
--- a/.php-cs-fixer.dist.php
+++ b/.php-cs-fixer.dist.php
@@ -16,6 +16,10 @@
         'native_function_invocation' => [
             'strict' => false
         ],
+        'nullable_type_declaration' => [
+            'syntax' => 'question_mark',
+        ],
+        'nullable_type_declaration_for_default_null_value' => true,
     ])
     ->setFinder(
         PhpCsFixer\Finder::create()
diff --git a/src/CachedKeySet.php b/src/CachedKeySet.php
index 65bab74f..8e8e8d68 100644
--- a/src/CachedKeySet.php
+++ b/src/CachedKeySet.php
@@ -80,9 +80,9 @@ public function __construct(
         ClientInterface $httpClient,
         RequestFactoryInterface $httpFactory,
         CacheItemPoolInterface $cache,
-        int $expiresAfter = null,
+        ?int $expiresAfter = null,
         bool $rateLimit = false,
-        string $defaultAlg = null
+        ?string $defaultAlg = null
     ) {
         $this->jwksUri = $jwksUri;
         $this->httpClient = $httpClient;
@@ -180,7 +180,7 @@ private function keyIdExists(string $keyId): bool
             $jwksResponse = $this->httpClient->sendRequest($request);
             if ($jwksResponse->getStatusCode() !== 200) {
                 throw new UnexpectedValueException(
-                    sprintf('HTTP Error: %d %s for URI "%s"',
+                    \sprintf('HTTP Error: %d %s for URI "%s"',
                         $jwksResponse->getStatusCode(),
                         $jwksResponse->getReasonPhrase(),
                         $this->jwksUri,
diff --git a/src/JWK.php b/src/JWK.php
index 63fb2484..6efc2fe3 100644
--- a/src/JWK.php
+++ b/src/JWK.php
@@ -52,7 +52,7 @@ class JWK
      *
      * @uses parseKey
      */
-    public static function parseKeySet(array $jwks, string $defaultAlg = null): array
+    public static function parseKeySet(array $jwks, ?string $defaultAlg = null): array
     {
         $keys = [];
 
@@ -93,7 +93,7 @@ public static function parseKeySet(array $jwks, string $defaultAlg = null): arra
      *
      * @uses createPemFromModulusAndExponent
      */
-    public static function parseKey(array $jwk, string $defaultAlg = null): ?Key
+    public static function parseKey(array $jwk, ?string $defaultAlg = null): ?Key
     {
         if (empty($jwk)) {
             throw new InvalidArgumentException('JWK must not be empty');
@@ -212,7 +212,7 @@ private static function createPemFromCrvAndXYCoordinates(string $crv, string $x,
                 )
             );
 
-        return sprintf(
+        return \sprintf(
             "-----BEGIN PUBLIC KEY-----\n%s\n-----END PUBLIC KEY-----\n",
             wordwrap(base64_encode($pem), 64, "\n", true)
         );
diff --git a/src/JWT.php b/src/JWT.php
index e9d75639..9100bf0f 100644
--- a/src/JWT.php
+++ b/src/JWT.php
@@ -96,7 +96,7 @@ class JWT
     public static function decode(
         string $jwt,
         $keyOrKeyArray,
-        stdClass &$headers = null
+        ?stdClass &$headers = null
     ): stdClass {
         // Validate JWT
         $timestamp = \is_null(static::$timestamp) ? \time() : static::$timestamp;
@@ -200,8 +200,8 @@ public static function encode(
         array $payload,
         $key,
         string $alg,
-        string $keyId = null,
-        array $head = null
+        ?string $keyId = null,
+        ?array $head = null
     ): string {
         $header = ['typ' => 'JWT'];
         if (isset($head) && \is_array($head)) {