diff -u b/core/lib/Drupal/Core/Entity/ContentEntityBase.php b/core/lib/Drupal/Core/Entity/ContentEntityBase.php
--- b/core/lib/Drupal/Core/Entity/ContentEntityBase.php
+++ b/core/lib/Drupal/Core/Entity/ContentEntityBase.php
@@ -160,7 +160,7 @@
    *
    * @var bool
    */
-  public $deepSerialization = FALSE;
+  protected $deepSerialization = FALSE;
 
   /**
    * {@inheritdoc}