diff --git a/core/lib/Drupal/Core/Template/Attribute.php b/core/lib/Drupal/Core/Template/Attribute.php
index a410610..cebe699 100644
--- a/core/lib/Drupal/Core/Template/Attribute.php
+++ b/core/lib/Drupal/Core/Template/Attribute.php
@@ -180,7 +180,8 @@ public function removeClass() {
         $classes = array_merge($classes, (array) $arg);
       }
 
-      // Remove the values passed in from the value array.
+      // Remove the values passed in from the value array. Use array_values() to
+      // ensure that the array index remains sequential.
       $classes = array_values(array_diff($this->storage['class']->value(), $classes));
       $this->storage['class']->exchangeArray($classes);
     }
diff --git a/core/tests/Drupal/Tests/Core/Template/AttributeTest.php b/core/tests/Drupal/Tests/Core/Template/AttributeTest.php
index a02f67b..b04eb51 100644
--- a/core/tests/Drupal/Tests/Core/Template/AttributeTest.php
+++ b/core/tests/Drupal/Tests/Core/Template/AttributeTest.php
@@ -133,6 +133,7 @@ public function testRemoveClasses() {
     // Remove a class that does not exist.
     $attribute->removeClass('gg');
     $this->assertNotContains(array('gg'), $attribute['class']->value());
+    // Test that the array index remains sequential.
     $this->assertArrayEquals(array('aa'), $attribute['class']->value());
 
     $attribute->removeClass('aa');