diff -u b/src/Plugin/field_group/FieldGroupFormatter/Fieldset.php b/src/Plugin/field_group/FieldGroupFormatter/Fieldset.php
--- b/src/Plugin/field_group/FieldGroupFormatter/Fieldset.php
+++ b/src/Plugin/field_group/FieldGroupFormatter/Fieldset.php
@@ -46,7 +46,11 @@
 
     $classes = $this->getClasses();
     if (!empty($classes)) {
-      $element['#attributes']['class'] = array_merge($element['#attributes']['class'], $classes);
+      if (isset($element['#attributes']['class'])) {
+        $element['#attributes']['class'] = array_merge($element['#attributes']['class'], $classes);
+      } else {
+        $element['#attributes'] += ['class' => $classes];
+      }
     }
 
     if ($this->getSetting('required_fields')) {
only in patch2:
unchanged:
--- a/field_group.libraries.yml
+++ b/field_group.libraries.yml
@@ -23,7 +23,7 @@ formatter.accordion:
   js:
     formatters/accordion/accordion.js: {}
   dependencies:
-    - core/jquery.ui.accordion
+    - jquery_ui_accordion/accordion
 
 formatter.html_element:
   js: