diff --git a/core/lib/Drupal/Core/Template/TwigSandboxPolicy.php b/core/lib/Drupal/Core/Template/TwigSandboxPolicy.php
index 5423bde..a7bb43f 100644
--- a/core/lib/Drupal/Core/Template/TwigSandboxPolicy.php
+++ b/core/lib/Drupal/Core/Template/TwigSandboxPolicy.php
@@ -81,8 +81,10 @@ public function checkPropertyAllowed($obj, $property) {}
    * {@inheritdoc}
    */
   public function checkMethodAllowed($obj, $method) {
-    if (isset($this->whitelisted_classes[get_class($obj)])) {
-      return TRUE;
+    foreach ($this->whitelisted_classes as $class => $key) {
+      if ($obj instanceof $class) {
+        return TRUE;
+      }
     }
 
     // Return quickly for an exact match of the method name.