diff --git a/src/CacheItemsTrait.php b/src/CacheItemsTrait.php
index 402d8ba..461feec 100644
--- a/src/CacheItemsTrait.php
+++ b/src/CacheItemsTrait.php
@@ -18,8 +18,8 @@
  */
 trait CacheItemsTrait
 {
-    static $CACHE_KEY_PREFIX = 'flysystem_item_';
-    static $CACHE_KEY_HASH_SALT = '563ce5132194441b';
+    static string $CACHE_KEY_PREFIX = 'flysystem_item_';
+    static string $CACHE_KEY_HASH_SALT = '563ce5132194441b';
 
     /** @var array<CacheItemInterface> */
     protected $cacheItems = [];