diff --git a/CompoundLimiter.php b/CompoundLimiter.php
index 1c4f560..1dc0854 100644
--- a/CompoundLimiter.php
+++ b/CompoundLimiter.php
@@ -31,7 +31,7 @@ public function __construct(array $limiters)
         $this->limiters = $limiters;
     }
 
-    public function reserve(int $tokens = 1, float $maxTime = null): Reservation
+    public function reserve(int $tokens = 1, ?float $maxTime = null): Reservation
     {
         throw new ReserveNotSupportedException(__CLASS__);
     }
diff --git a/Exception/MaxWaitDurationExceededException.php b/Exception/MaxWaitDurationExceededException.php
index 53029ac..25a868d 100644
--- a/Exception/MaxWaitDurationExceededException.php
+++ b/Exception/MaxWaitDurationExceededException.php
@@ -20,7 +20,7 @@ class MaxWaitDurationExceededException extends \RuntimeException
 {
     private RateLimit $rateLimit;
 
-    public function __construct(string $message, RateLimit $rateLimit, int $code = 0, \Throwable $previous = null)
+    public function __construct(string $message, RateLimit $rateLimit, int $code = 0, ?\Throwable $previous = null)
     {
         parent::__construct($message, $code, $previous);
 
diff --git a/Exception/RateLimitExceededException.php b/Exception/RateLimitExceededException.php
index adbe576..15b52b9 100644
--- a/Exception/RateLimitExceededException.php
+++ b/Exception/RateLimitExceededException.php
@@ -20,7 +20,7 @@ class RateLimitExceededException extends \RuntimeException
 {
     private RateLimit $rateLimit;
 
-    public function __construct(RateLimit $rateLimit, int $code = 0, \Throwable $previous = null)
+    public function __construct(RateLimit $rateLimit, int $code = 0, ?\Throwable $previous = null)
     {
         parent::__construct('Rate Limit Exceeded', $code, $previous);
 
diff --git a/Exception/ReserveNotSupportedException.php b/Exception/ReserveNotSupportedException.php
index cb7a306..07a0fac 100644
--- a/Exception/ReserveNotSupportedException.php
+++ b/Exception/ReserveNotSupportedException.php
@@ -16,7 +16,7 @@
  */
 class ReserveNotSupportedException extends \BadMethodCallException
 {
-    public function __construct(string $limiterClass, int $code = 0, \Throwable $previous = null)
+    public function __construct(string $limiterClass, int $code = 0, ?\Throwable $previous = null)
     {
         parent::__construct(sprintf('Reserving tokens is not supported by "%s".', $limiterClass), $code, $previous);
     }
diff --git a/LimiterInterface.php b/LimiterInterface.php
index 6f0ae7d..f95cf8c 100644
--- a/LimiterInterface.php
+++ b/LimiterInterface.php
@@ -33,7 +33,7 @@ interface LimiterInterface
      * @throws ReserveNotSupportedException     if this limiter implementation doesn't support reserving tokens
      * @throws \InvalidArgumentException        if $tokens is larger than the maximum burst size
      */
-    public function reserve(int $tokens = 1, float $maxTime = null): Reservation;
+    public function reserve(int $tokens = 1, ?float $maxTime = null): Reservation;
 
     /**
      * Use this method if you intend to drop if the required number
diff --git a/Policy/FixedWindowLimiter.php b/Policy/FixedWindowLimiter.php
index 6e71fe2..1d116e0 100644
--- a/Policy/FixedWindowLimiter.php
+++ b/Policy/FixedWindowLimiter.php
@@ -29,7 +29,7 @@ final class FixedWindowLimiter implements LimiterInterface
     private int $limit;
     private int $interval;
 
-    public function __construct(string $id, int $limit, \DateInterval $interval, StorageInterface $storage, LockInterface $lock = null)
+    public function __construct(string $id, int $limit, \DateInterval $interval, StorageInterface $storage, ?LockInterface $lock = null)
     {
         if ($limit < 1) {
             throw new \InvalidArgumentException(sprintf('Cannot set the limit of "%s" to 0, as that would never accept any hit.', __CLASS__));
@@ -42,7 +42,7 @@ public function __construct(string $id, int $limit, \DateInterval $interval, Sto
         $this->interval = TimeUtil::dateIntervalToSeconds($interval);
     }
 
-    public function reserve(int $tokens = 1, float $maxTime = null): Reservation
+    public function reserve(int $tokens = 1, ?float $maxTime = null): Reservation
     {
         if ($tokens > $this->limit) {
             throw new \InvalidArgumentException(sprintf('Cannot reserve more tokens (%d) than the size of the rate limiter (%d).', $tokens, $this->limit));
diff --git a/Policy/NoLimiter.php b/Policy/NoLimiter.php
index 4878e4a..56339d3 100644
--- a/Policy/NoLimiter.php
+++ b/Policy/NoLimiter.php
@@ -25,7 +25,7 @@
  */
 final class NoLimiter implements LimiterInterface
 {
-    public function reserve(int $tokens = 1, float $maxTime = null): Reservation
+    public function reserve(int $tokens = 1, ?float $maxTime = null): Reservation
     {
         return new Reservation(microtime(true), new RateLimit(\PHP_INT_MAX, new \DateTimeImmutable(), true, \PHP_INT_MAX));
     }
diff --git a/Policy/SlidingWindowLimiter.php b/Policy/SlidingWindowLimiter.php
index 468b0d0..fc9173d 100644
--- a/Policy/SlidingWindowLimiter.php
+++ b/Policy/SlidingWindowLimiter.php
@@ -37,7 +37,7 @@ final class SlidingWindowLimiter implements LimiterInterface
     private int $limit;
     private int $interval;
 
-    public function __construct(string $id, int $limit, \DateInterval $interval, StorageInterface $storage, LockInterface $lock = null)
+    public function __construct(string $id, int $limit, \DateInterval $interval, StorageInterface $storage, ?LockInterface $lock = null)
     {
         $this->storage = $storage;
         $this->lock = $lock;
@@ -46,7 +46,7 @@ public function __construct(string $id, int $limit, \DateInterval $interval, Sto
         $this->interval = TimeUtil::dateIntervalToSeconds($interval);
     }
 
-    public function reserve(int $tokens = 1, float $maxTime = null): Reservation
+    public function reserve(int $tokens = 1, ?float $maxTime = null): Reservation
     {
         if ($tokens > $this->limit) {
             throw new \InvalidArgumentException(sprintf('Cannot reserve more tokens (%d) than the size of the rate limiter (%d).', $tokens, $this->limit));
diff --git a/Policy/TokenBucket.php b/Policy/TokenBucket.php
index 517c669..f390599 100644
--- a/Policy/TokenBucket.php
+++ b/Policy/TokenBucket.php
@@ -32,7 +32,7 @@ final class TokenBucket implements LimiterStateInterface
      * @param Rate       $rate          the fill rate and time of this bucket
      * @param float|null $timer         the current timer of the bucket, defaulting to microtime(true)
      */
-    public function __construct(string $id, int $initialTokens, Rate $rate, float $timer = null)
+    public function __construct(string $id, int $initialTokens, Rate $rate, ?float $timer = null)
     {
         if ($initialTokens < 1) {
             throw new \InvalidArgumentException(sprintf('Cannot set the limit of "%s" to 0, as that would never accept any hit.', TokenBucketLimiter::class));
diff --git a/Policy/TokenBucketLimiter.php b/Policy/TokenBucketLimiter.php
index d1ebeb2..dec4720 100644
--- a/Policy/TokenBucketLimiter.php
+++ b/Policy/TokenBucketLimiter.php
@@ -28,7 +28,7 @@ final class TokenBucketLimiter implements LimiterInterface
     private int $maxBurst;
     private Rate $rate;
 
-    public function __construct(string $id, int $maxBurst, Rate $rate, StorageInterface $storage, LockInterface $lock = null)
+    public function __construct(string $id, int $maxBurst, Rate $rate, StorageInterface $storage, ?LockInterface $lock = null)
     {
         $this->id = $id;
         $this->maxBurst = $maxBurst;
@@ -50,7 +50,7 @@ public function __construct(string $id, int $maxBurst, Rate $rate, StorageInterf
      * @throws MaxWaitDurationExceededException if $maxTime is set and the process needs to wait longer than its value (in seconds)
      * @throws \InvalidArgumentException        if $tokens is larger than the maximum burst size
      */
-    public function reserve(int $tokens = 1, float $maxTime = null): Reservation
+    public function reserve(int $tokens = 1, ?float $maxTime = null): Reservation
     {
         if ($tokens > $this->maxBurst) {
             throw new \InvalidArgumentException(sprintf('Cannot reserve more tokens (%d) than the burst size of the rate limiter (%d).', $tokens, $this->maxBurst));
diff --git a/Policy/Window.php b/Policy/Window.php
index 42d00de..6a103fc 100644
--- a/Policy/Window.php
+++ b/Policy/Window.php
@@ -26,7 +26,7 @@ final class Window implements LimiterStateInterface
     private int $maxSize;
     private float $timer;
 
-    public function __construct(string $id, int $intervalInSeconds, int $windowSize, float $timer = null)
+    public function __construct(string $id, int $intervalInSeconds, int $windowSize, ?float $timer = null)
     {
         $this->id = $id;
         $this->intervalInSeconds = $intervalInSeconds;
@@ -44,7 +44,7 @@ public function getExpirationTime(): ?int
         return $this->intervalInSeconds;
     }
 
-    public function add(int $hits = 1, float $now = null): void
+    public function add(int $hits = 1, ?float $now = null): void
     {
         $now ??= microtime(true);
         if (($now - $this->timer) > $this->intervalInSeconds) {
diff --git a/RateLimiterFactory.php b/RateLimiterFactory.php
index 1b397ac..e9afcb0 100644
--- a/RateLimiterFactory.php
+++ b/RateLimiterFactory.php
@@ -30,7 +30,7 @@ final class RateLimiterFactory
     private StorageInterface $storage;
     private ?LockFactory $lockFactory;
 
-    public function __construct(array $config, StorageInterface $storage, LockFactory $lockFactory = null)
+    public function __construct(array $config, StorageInterface $storage, ?LockFactory $lockFactory = null)
     {
         $this->storage = $storage;
         $this->lockFactory = $lockFactory;
@@ -41,7 +41,7 @@ public function __construct(array $config, StorageInterface $storage, LockFactor
         $this->config = $options->resolve($config);
     }
 
-    public function create(string $key = null): LimiterInterface
+    public function create(?string $key = null): LimiterInterface
     {
         $id = $this->config['id'].'-'.$key;
         $lock = $this->lockFactory?->createLock($id);
diff --git a/Tests/Policy/TokenBucketLimiterTest.php b/Tests/Policy/TokenBucketLimiterTest.php
index 94a8ca1..8ed21c2 100644
--- a/Tests/Policy/TokenBucketLimiterTest.php
+++ b/Tests/Policy/TokenBucketLimiterTest.php
@@ -182,7 +182,7 @@ public function testBucketRefilledWithStrictFrequency()
         }
     }
 
-    private function createLimiter($initialTokens = 10, Rate $rate = null)
+    private function createLimiter($initialTokens = 10, ?Rate $rate = null)
     {
         return new TokenBucketLimiter('test', $initialTokens, $rate ?? Rate::perSecond(10), $this->storage);
     }