diff -u b/core/lib/Drupal/Core/Template/Attribute.php b/core/lib/Drupal/Core/Template/Attribute.php
--- b/core/lib/Drupal/Core/Template/Attribute.php
+++ b/core/lib/Drupal/Core/Template/Attribute.php
@@ -334,7 +334,7 @@
    */
   #[\ReturnTypeWillChange]
   public function count() {
-    return count($this->__toString());
+    return count($this->toString());
   }
 
   /**