diff --git a/src/Widget/WidgetPluginBase.php b/src/Widget/WidgetPluginBase.php
index 30b4f63d828c2944fdad135be3aa9351c8127b8c..bd0987fa3fc26784924b78730c9a4ab719ac67e0 100644
--- a/src/Widget/WidgetPluginBase.php
+++ b/src/Widget/WidgetPluginBase.php
@@ -198,7 +198,7 @@ abstract class WidgetPluginBase extends PluginBase implements WidgetPluginInterf
     }
 
     if ($result->isActive()) {
-      $items['#attributes'] = ['class' => ['is-active']];
+      $items['#attributes']['class'][] = 'is-active';
     }
 
     $items['#wrapper_attributes'] = ['class' => $classes];