diff --git a/core/modules/aggregator/lib/Drupal/aggregator/Form/SettingsForm.php b/core/modules/aggregator/lib/Drupal/aggregator/Form/SettingsForm.php
index 6abd8d9..81e1b39 100644
--- a/core/modules/aggregator/lib/Drupal/aggregator/Form/SettingsForm.php
+++ b/core/modules/aggregator/lib/Drupal/aggregator/Form/SettingsForm.php
@@ -104,6 +104,8 @@ public function getFormID() {
    */
   protected function getConfigurableInstances() {
     if (!isset($this->configurableInstances)) {
+      $this->configurableInstances = array();
+
       $config = $this->configFactory->get('aggregator.settings');
 
       // Parse fetchers and parsers and collect their PluginFormInterface