diff --git a/src/Widget/WidgetPluginBase.php b/src/Widget/WidgetPluginBase.php
index e33ed28..72db436 100644
--- a/src/Widget/WidgetPluginBase.php
+++ b/src/Widget/WidgetPluginBase.php
@@ -164,7 +164,7 @@ 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];