diff --git a/core/tests/Drupal/Tests/Core/Template/AttributeTest.php b/core/tests/Drupal/Tests/Core/Template/AttributeTest.php
index f69102e862..9c245f16fb 100644
--- a/core/tests/Drupal/Tests/Core/Template/AttributeTest.php
+++ b/core/tests/Drupal/Tests/Core/Template/AttributeTest.php
@@ -531,6 +531,8 @@ public function testCount() {
     $this->assertSame(0, count($attributes));
     $attributes['class'][] = 'cat';
     $this->assertSame(1, count($attributes));
+    $attributes['class'][] = 'tabby';
+    $this->assertSame(1, count($attributes));
     $attributes['id'] = 'test_id';
     $this->assertSame(2, count($attributes));
     $attributes['class'] = [];