diff --git a/libs/sysplugins/smarty_internal_cacheresource_file.php b/libs/sysplugins/smarty_internal_cacheresource_file.php
index abed98d8..c77ae9e1 100644
--- a/libs/sysplugins/smarty_internal_cacheresource_file.php
+++ b/libs/sysplugins/smarty_internal_cacheresource_file.php
@@ -196,8 +196,8 @@ class Smarty_Internal_CacheResource_File extends Smarty_CacheResource
      */
     public function hasLock(Smarty $smarty, Smarty_Template_Cached $cached)
     {
-        clearstatcache(true, $cached->lock_id);
-        if (is_file($cached->lock_id)) {
+        clearstatcache(true, $cached->lock_id ?? '');
+        if (null !== $cached->lock_id && is_file($cached->lock_id)) {
             $t = filemtime($cached->lock_id);
             return $t && (time() - $t < $smarty->locking_timeout);
         } else {