diff --git a/core/lib/Drupal/Core/Field/PluginSettingsBase.php b/core/lib/Drupal/Core/Field/PluginSettingsBase.php
index e69393a..05484f8 100644
--- a/core/lib/Drupal/Core/Field/PluginSettingsBase.php
+++ b/core/lib/Drupal/Core/Field/PluginSettingsBase.php
@@ -52,8 +52,8 @@ public function getSettings() {
    * {@inheritdoc}
    */
   public function getSetting($key) {
-    // Merge defaults if we have no value for the key.
-    if (!$this->defaultSettingsMerged && !array_key_exists($key, $this->settings)) {
+    // Merge defaults if that has not been done before.
+    if (!$this->defaultSettingsMerged) {
       $this->mergeDefaults();
     }
     return isset($this->settings[$key]) ? $this->settings[$key] : NULL;
@@ -63,7 +63,8 @@ public function getSetting($key) {
    * Merges default settings values into $settings.
    */
   protected function mergeDefaults() {
-    $this->settings += static::settings();
+    // Use only settings defined by plugin.
+    $this->settings = array_intersect_key($this->settings, static::settings()) + static::settings();
     $this->defaultSettingsMerged = TRUE;
   }
 
diff --git a/core/modules/field_ui/lib/Drupal/field_ui/Tests/ManageDisplayTest.php b/core/modules/field_ui/lib/Drupal/field_ui/Tests/ManageDisplayTest.php
index e189c2b..971d7cc 100644
--- a/core/modules/field_ui/lib/Drupal/field_ui/Tests/ManageDisplayTest.php
+++ b/core/modules/field_ui/lib/Drupal/field_ui/Tests/ManageDisplayTest.php
@@ -106,9 +106,9 @@ function testFormatterUI() {
     $edit = array($fieldname => 'foo');
     $this->drupalPostAjaxForm(NULL, $edit, "field_test_plugin_settings_update");
 
-    // Confirm that the settings are updated on the settings form.
+    // Confirm that the extra settings are not updated on the settings form.
     $this->drupalPostAjaxForm(NULL, array(), "field_test_settings_edit");
-    $this->assertFieldByName($fieldname, 'foo');
+    $this->assertNoFieldByName($fieldname, 'foo');
 
     // Test the empty setting formatter.
     $edit = array('fields[field_test][type]' => 'field_empty_setting');