diff --git a/core/lib/Drupal/Core/Config/Entity/ConfigEntityBase.php b/core/lib/Drupal/Core/Config/Entity/ConfigEntityBase.php
index f366900..330f39c 100644
--- a/core/lib/Drupal/Core/Config/Entity/ConfigEntityBase.php
+++ b/core/lib/Drupal/Core/Config/Entity/ConfigEntityBase.php
@@ -32,6 +32,7 @@ public function __construct(array $values, $entity_type) {
     parent::__construct($values, $entity_type);
 
     // Backup the original ID, if any.
+    // Configuration entity IDs are strings, and '0' is a valid ID.
     $original_id = $this->id();
     if ($original_id !== NULL && $original_id !== '') {
       $this->setOriginalID($original_id);
@@ -60,6 +61,7 @@ public function setOriginalID($id) {
    * configuration entity is unique.
    */
   final public function isNew() {
+    // Configuration entity IDs are strings, and '0' is a valid ID.
     return !empty($this->enforceIsNew) || $this->id() === NULL || $this->id() === '';
   }
 
diff --git a/core/lib/Drupal/Core/Config/Entity/ConfigStorageController.php b/core/lib/Drupal/Core/Config/Entity/ConfigStorageController.php
index eb65f5d..f37acd3 100644
--- a/core/lib/Drupal/Core/Config/Entity/ConfigStorageController.php
+++ b/core/lib/Drupal/Core/Config/Entity/ConfigStorageController.php
@@ -225,9 +225,6 @@ public function create(array $values) {
     $entity = new $class($values, $this->entityType);
     // Mark this entity as new, so isNew() returns TRUE. This does not check
     // whether a configuration entity with the same ID (if any) already exists.
-    // @todo If we expect isNew() to return FALSE when given an ID that exists
-    //   already, wrap this in !config($prefix . $id)->getStorage()->exists().
-    //   In that case, do we expect existing values to be merged into $values?
     $entity->enforceIsNew();
 
     // Assign a new UUID if there is none yet.
@@ -274,12 +271,14 @@ public function delete($ids) {
   public function save(EntityInterface $entity) {
     $prefix = $this->entityInfo['config prefix'] . '.';
 
+    // Configuration entity IDs are strings, and '0' is a valid ID.
     $id = $entity->id();
     if ($id === NULL || $id === '') {
       throw new EntityMalformedException('The entity does not have an ID.');
     }
 
     // Load the stored entity, if any.
+    // At this point, the original ID can only be NULL or a valid ID.
     if ($entity->getOriginalID() !== NULL) {
       $id = $entity->getOriginalID();
     }