diff --git a/core/lib/Drupal/Core/Config/Entity/ConfigEntityBase.php b/core/lib/Drupal/Core/Config/Entity/ConfigEntityBase.php
index d2f26f7..0abb9a7 100644
--- a/core/lib/Drupal/Core/Config/Entity/ConfigEntityBase.php
+++ b/core/lib/Drupal/Core/Config/Entity/ConfigEntityBase.php
@@ -375,17 +375,23 @@ public function getConfigDependencyName() {
    * {@inheritdoc}
    */
   public function serialize() {
-    $values = $this->toArray();
-    $values['entityTypeID'] = $this->getEntityTypeId();
-    return serialize($values);
+    return serialize(array(
+      'entity_type_id' => $this->getEntityTypeId(),
+      'values' => $this->toArray(),
+    ));
   }
 
   /**
    * {@inheritdoc}
    */
   public function unserialize($serialized) {
-    $values = unserialize($serialized);
-    return $this->__construct($values, $values['entityTypeID']);
+    // PHPUnit uses unserialize() on a made up string as a trick when bypassing
+    // the contructor on mock object creation. Make sure we only run on objects
+    // that have been serialized through our serialize() method.
+    $unserialized = unserialize($serialized);
+    if (isset($unserialized['entity_type_id'])) {
+      return $this->__construct($unserialized['values'], $unserialized['entity_type_id']);
+    }
   }
 
 }