diff --git a/core/lib/Drupal/Core/Entity/Entity.php b/core/lib/Drupal/Core/Entity/Entity.php
index 785c81c..1eb7c23 100644
--- a/core/lib/Drupal/Core/Entity/Entity.php
+++ b/core/lib/Drupal/Core/Entity/Entity.php
@@ -331,6 +331,10 @@ public function getTranslationLanguages($include_default = TRUE) {
    * Implements \Drupal\Core\Entity\EntityInterface::save().
    */
   public function save() {
+    // Entities that are not new must have UUIDs.
+    if (!$this->isNew() && !$this->uuid()) {
+      throw new EntityMalformedException('Only new entities can be saved without a UUID.');
+    }
     return drupal_container()->get('plugin.manager.entity')->getStorageController($this->entityType)->save($this);
   }