diff -u b/cdn_ui/src/Form/CdnSettingsForm.php b/cdn_ui/src/Form/CdnSettingsForm.php
--- b/cdn_ui/src/Form/CdnSettingsForm.php
+++ b/cdn_ui/src/Form/CdnSettingsForm.php
@@ -205,9 +205,11 @@
     $form['wrappers']['stream_wrappers'] += $checkboxes;
     // Special cases: public:// and private://.
     $form['wrappers']['stream_wrappers']['public']['#disabled'] = TRUE;
-    $form['wrappers']['stream_wrappers']['private']['#disabled'] = TRUE;
-    $form['wrappers']['stream_wrappers']['private']['#title'] = '<del>' . $form['wrappers']['stream_wrappers']['private']['#title'] . '</del>';
-    $form['wrappers']['stream_wrappers']['private']['#description'] = $this->t('Private files require authentication and hence cannot be served from a CDN.');
+    if (!empty($form['wrappers']['stream_wrappers']['private'])) {
+      $form['wrappers']['stream_wrappers']['private']['#disabled'] = TRUE;
+      $form['wrappers']['stream_wrappers']['private']['#title'] = '<del>' . $form['wrappers']['stream_wrappers']['private']['#title'] . '</del>';
+      $form['wrappers']['stream_wrappers']['private']['#description'] = $this->t('Private files require authentication and hence cannot be served from a CDN.');
+    }
 
     return parent::buildForm($form, $form_state);
   }