diff --git a/core/lib/Drupal/Core/Config/Config.php b/core/lib/Drupal/Core/Config/Config.php
index 02afbd1..97fbb4f 100644
--- a/core/lib/Drupal/Core/Config/Config.php
+++ b/core/lib/Drupal/Core/Config/Config.php
@@ -398,12 +398,13 @@ public function castValue($value) {
    * Unsets value in this config object.
    *
    * @param string $key
-   *   Name of the key whose value should be unset.
+   *   (optional) The name of the key whose value should be unset. If omitted,
+   *   the entire config object is emptied.
    *
    * @return Drupal\Core\Config\Config
    *   The configuration object.
    */
-  public function clear($key) {
+  public function clear($key = NULL) {
     if (!$this->isLoaded) {
       $this->load();
     }
diff --git a/core/lib/Drupal/Core/Config/Entity/ConfigStorageController.php b/core/lib/Drupal/Core/Config/Entity/ConfigStorageController.php
index d8e2ed5..fa90869 100644
--- a/core/lib/Drupal/Core/Config/Entity/ConfigStorageController.php
+++ b/core/lib/Drupal/Core/Config/Entity/ConfigStorageController.php
@@ -410,6 +410,10 @@ public function save(EntityInterface $entity) {
     $entity->preSave($this);
     $this->invokeHook('presave', $entity);
 
+    // Clear out any possibly existing keys in an existing configuration object,
+    // so any potentially stale keys are removed.
+    $config->clear();
+
     // Retrieve the desired properties and set them in config.
     foreach ($entity->getExportProperties() as $key => $value) {
       $config->set($key, $value);