diff --git a/src/Mutex/RedisServerMutex.php b/src/Mutex/RedisServerMutex.php
index aa3392d..bce4510 100644
--- a/src/Mutex/RedisServerMutex.php
+++ b/src/Mutex/RedisServerMutex.php
@@ -56,7 +56,7 @@ public function __construct(
         $this->retryInterval = (int) ($options['retry_interval'] ?? 10);
     }
 
-    public function attempt(callable $callback = null): void
+    public function attempt(?callable $callback = null): void
     {
         // Waiting for the server mutex.
         $this->timer->tick($this->retryInterval, function () {
diff --git a/src/Mutex/ServerMutexInterface.php b/src/Mutex/ServerMutexInterface.php
index 6afe143..208b2e9 100644
--- a/src/Mutex/ServerMutexInterface.php
+++ b/src/Mutex/ServerMutexInterface.php
@@ -13,7 +13,7 @@
 
 interface ServerMutexInterface
 {
-    public function attempt(callable $callback = null);
+    public function attempt(?callable $callback = null);
 
     public function release(bool $force = false);
 }