diff --git a/core/lib/Drupal/Core/Template/Attribute.php b/core/lib/Drupal/Core/Template/Attribute.php
index 63f21c2d1626cce1e41e9233c84907bdab261b20..7422e26f463dc42b864dd937ea728e68ab41fd7e 100644
--- a/core/lib/Drupal/Core/Template/Attribute.php
+++ b/core/lib/Drupal/Core/Template/Attribute.php
@@ -94,6 +94,13 @@ public function offsetGet($name): mixed {
     if (isset($this->storage[$name])) {
       return $this->storage[$name];
     }
+    // The 'class' array key is expected to be itself an array, and therefore
+    // can be accessed using array append syntax before it has been initialized.
+    if ($name === 'class') {
+      // Initialize the class attribute as an empty array if not set.
+      $this->offsetSet('class', []);
+      return $this->storage['class'];
+    }
     return NULL;
   }
 
diff --git a/core/tests/Drupal/Tests/Core/Template/AttributeTest.php b/core/tests/Drupal/Tests/Core/Template/AttributeTest.php
index f76bc92c757649e9116694148c7c2a120e283c6a..46ac4daf36dfea5e86614f1cd6561f0ce8a69d07 100644
--- a/core/tests/Drupal/Tests/Core/Template/AttributeTest.php
+++ b/core/tests/Drupal/Tests/Core/Template/AttributeTest.php
@@ -148,12 +148,16 @@ public function testRemoveAttribute(): void {
    * @covers ::addClass
    */
   public function testAddClasses(): void {
-    // Add empty Attribute object with no classes.
+    // Add a class with the array syntax without first initializing the 'class'
+    // attribute.
     $attribute = new Attribute();
+    $attribute['class'][] = 'test-class';
+    $this->assertEquals(new AttributeArray('class', ['test-class']), $attribute['class']);
 
+    $attribute = new Attribute();
     // Add no class on empty attribute.
     $attribute->addClass();
-    $this->assertEmpty($attribute['class']);
+    $this->assertEmpty($attribute['class']->value());
 
     // Test various permutations of adding values to empty Attribute objects.
     foreach ([NULL, FALSE, '', []] as $value) {