diff --git a/src/Registry/CachedRegistry.php b/src/Registry/CachedRegistry.php
index 281e193..9b5b507 100644
--- a/src/Registry/CachedRegistry.php
+++ b/src/Registry/CachedRegistry.php
@@ -32,7 +32,7 @@ class CachedRegistry implements Registry
      */
     private $hashAlgoFunc;
 
-    public function __construct(Registry $registry, CacheAdapter $cacheAdapter, callable $hashAlgoFunc = null)
+    public function __construct(Registry $registry, CacheAdapter $cacheAdapter, ?callable $hashAlgoFunc = null)
     {
         $this->registry = $registry;
         $this->cacheAdapter = $cacheAdapter;