diff --git a/core/lib/Drupal/Core/Template/Attribute.php b/core/lib/Drupal/Core/Template/Attribute.php
index abd889a..d64824d 100644
--- a/core/lib/Drupal/Core/Template/Attribute.php
+++ b/core/lib/Drupal/Core/Template/Attribute.php
@@ -143,9 +143,10 @@ public function addClass() {
         // Merge the values passed in from the classes array.
         // The argument is cast to an array to support comma separated single
         // values or one or more array arguments.
-        // We also filter out any empty values from the array.
-        $classes = array_filter(array_merge($classes, (array) $arg));
+        $classes = array_merge($classes, (array) $arg);
       }
+      // Filter out any empty values from the array.
+      $classes = array_filter($classes);
       // If there are no classes to add, return early.
       if (empty($classes)) {
         return $this;