diff --git a/src/EtcdLock.php b/src/EtcdLock.php
index 89f774f..17b735f 100644
--- a/src/EtcdLock.php
+++ b/src/EtcdLock.php
@@ -25,7 +25,7 @@ class EtcdLock extends Lock implements LockInterface
      * @throws InvalidResponseStatusCodeException
      * @throws TooManySaveRetriesException
      */
-    public function __construct(string $key, bool $exclusive = false, int $time = 120, int $wait = 300, string $identifier = null)
+    public function __construct(string $key, bool $exclusive = false, int $time = 120, int $wait = 300, ?string $identifier = null)
     {
         parent::__construct($key);
         $this->lock($exclusive, $time, $wait, $identifier);
diff --git a/src/Lock.php b/src/Lock.php
index 456cb2e..a545eea 100644
--- a/src/Lock.php
+++ b/src/Lock.php
@@ -263,7 +263,7 @@ public function __construct(string $key)
      * @throws InvalidResponseStatusCodeException
      * @throws TooManySaveRetriesException
      */
-    public function lock(bool $exclusive = false, int $time = 120, int $wait = 300, string $identifier = null): bool
+    public function lock(bool $exclusive = false, int $time = 120, int $wait = 300, ?string $identifier = null): bool
     {
         $this->exclusive = $exclusive;
         $this->time = $time;
diff --git a/src/LockInterface.php b/src/LockInterface.php
index 7f5e8b7..9beaba2 100644
--- a/src/LockInterface.php
+++ b/src/LockInterface.php
@@ -18,7 +18,7 @@ interface LockInterface
      * @param int $wait
      * @param string|null $identifier
      */
-    public function __construct(string $key, bool $exclusive = false, int $time = 60, int $wait = 300, string $identifier = null);
+    public function __construct(string $key, bool $exclusive = false, int $time = 60, int $wait = 300, ?string $identifier = null);
 
     /**
      * Check if is locked and returns time until lock runs out or false