diff --git a/src/ClassyLayout.php b/src/ClassyLayout.php
index ea71efa..a889d5d 100644
--- a/src/ClassyLayout.php
+++ b/src/ClassyLayout.php
@@ -66,7 +66,7 @@ class ClassyLayout extends LayoutDefault implements PluginFormInterface {
       if (empty($class_definition['options']) || !is_array($class_definition['options'])) {
         throw new \Exception('The "options" key is required for layout class definitions.');
       }
-      $definition_default = $class_definition['default'] ?? NULL;
+      $definition_default = $class_definition['default'] ?? [];
       $form['classes'][$key] = [
         '#title' => $class_definition['label'] ?? $this->t('Classes'),
         '#type' => 'select',