diff --git a/core/lib/Drupal/Core/Field/PluginSettingsBase.php b/core/lib/Drupal/Core/Field/PluginSettingsBase.php
index b5aeff7..38d4233 100644
--- a/core/lib/Drupal/Core/Field/PluginSettingsBase.php
+++ b/core/lib/Drupal/Core/Field/PluginSettingsBase.php
@@ -63,7 +63,7 @@ public function getSetting($key) {
    * Merges default settings values into $settings.
    */
   protected function mergeDefaults() {
-    // Use only settings defined by plugin.
+    // Use only a settings defined by plugin.
     $this->settings += static::defaultSettings();
     $this->defaultSettingsMerged = TRUE;
   }
diff --git a/core/lib/Drupal/Core/Field/PluginSettingsInterface.php b/core/lib/Drupal/Core/Field/PluginSettingsInterface.php
index a289ce8..9f9b7c6 100644
--- a/core/lib/Drupal/Core/Field/PluginSettingsInterface.php
+++ b/core/lib/Drupal/Core/Field/PluginSettingsInterface.php
@@ -48,8 +48,7 @@ public function getSetting($key);
    *   The array of settings, keyed by setting names. Missing settings will be
    *   assigned their default values.
    *
-   * @return \Drupal\Core\Field\PluginSettingsInterface
-   *   The plugin itself.
+   * @return $this
    */
   public function setSettings(array $settings);
 
@@ -61,8 +60,7 @@ public function setSettings(array $settings);
    * @param mixed $value
    *   The setting value.
    *
-   * @return \Drupal\Core\Field\PluginSettingsInterface
-   *   The plugin itself.
+   * @return $this
    */
   public function setSetting($key, $value);
 
diff --git a/core/modules/options/lib/Drupal/options/Plugin/Field/FieldType/ListBooleanItem.php b/core/modules/options/lib/Drupal/options/Plugin/Field/FieldType/ListBooleanItem.php
index 81b7da5..a521459 100644
--- a/core/modules/options/lib/Drupal/options/Plugin/Field/FieldType/ListBooleanItem.php
+++ b/core/modules/options/lib/Drupal/options/Plugin/Field/FieldType/ListBooleanItem.php
@@ -30,6 +30,16 @@ class ListBooleanItem extends FieldItemBase implements AllowedValuesInterface {
   /**
    * {@inheritdoc}
    */
+  public static function defaultSettings() {
+    $settings = parent::defaultSettings();
+    $settings['allowed_values'] = array();
+    $settings['allowed_values_function'] = '';
+    return $settings;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
   public function getPossibleValues(AccountInterface $account = NULL) {
     return array_keys($this->getSettableOptions($account));
   }