diff --git a/core/lib/Drupal/Core/Entity/ContentEntityInterface.php b/core/lib/Drupal/Core/Entity/ContentEntityInterface.php
index d045152..ad38b9d3 100644
--- a/core/lib/Drupal/Core/Entity/ContentEntityInterface.php
+++ b/core/lib/Drupal/Core/Entity/ContentEntityInterface.php
@@ -71,4 +71,34 @@ public function getLoadedRevisionId();
    */
   public function updateLoadedRevisionId();
 
+  /**
+   * Flags the entity for deep serialization.
+   *
+   * When an entity is deeply serialized then its field items will have the
+   * opportunity to return also e.g. computed properties in order to serialize
+   * the whole entity structure and cover changes made on computed properties,
+   * which are not yet saved instead of only returning the metadata needed to
+   * re-compute/load the computed property. An example for this are entity
+   * reference fields, which during deep serialization will return their
+   * referenced entities. In this case not only the changes on the main entity
+   * will be serialized, but also the changes made on the referenced entities.
+   *
+   * Example usage:
+   * -Alter the title of a referenced entity and serialize the parent entity
+   * through deep serialization in order to serialize the change made on the
+   * referenced entity.
+   * @code
+   *   $entity->entity_reference_field->entity->setTitle('new entity reference title');
+   *   $entity->setDeepSerialization(TRUE);
+   *   serialize($entity);
+   * @endcode
+   *
+   * @param bool $deep_serialization
+   *   TRUE if deep serialization should be performed when the entity is being
+   *   serialized, FALSE otherwise. Defaults to TRUE.
+   *
+   * @return $this
+   */
+  public function setDeepSerialization($deep_serialization);
+
 }
diff --git a/core/lib/Drupal/Core/Entity/FieldableEntityInterface.php b/core/lib/Drupal/Core/Entity/FieldableEntityInterface.php
index 0d6b210..d21a2a8 100644
--- a/core/lib/Drupal/Core/Entity/FieldableEntityInterface.php
+++ b/core/lib/Drupal/Core/Entity/FieldableEntityInterface.php
@@ -236,34 +236,4 @@ public function isValidationRequired();
    */
   public function setValidationRequired($required);
 
-  /**
-   * Flags the entity for deep serialization.
-   *
-   * When an entity is deeply serialized then its field items will have the
-   * opportunity to return also e.g. computed properties in order to serialize
-   * the whole entity structure and cover changes made on computed properties,
-   * which are not yet saved instead of only returning the metadata needed to
-   * re-compute/load the computed property. An example for this are entity
-   * reference fields, which during deep serialization will return their
-   * referenced entities. In this case not only the changes on the main entity
-   * will be serialized, but also the changes made on the referenced entities.
-   *
-   * Example usage:
-   * -Alter the title of a referenced entity and serialize the parent entity
-   * through deep serialization in order to serialize the change made on the
-   * referenced entity.
-   * @code
-   *   $entity->entity_reference_field->entity->setTitle('new entity reference title');
-   *   $entity->setDeepSerialization(TRUE);
-   *   serialize($entity);
-   * @endcode
-   *
-   * @param bool $deep_serialization
-   *   TRUE if deep serialization should be performed when the entity is being
-   *   serialized, FALSE otherwise. Defaults to TRUE.
-   *
-   * @return $this
-   */
-  public function setDeepSerialization($deep_serialization);
-
 }
diff --git a/core/lib/Drupal/Core/Field/EntityReferenceFieldItemList.php b/core/lib/Drupal/Core/Field/EntityReferenceFieldItemList.php
index 6dc2ee6..f4fc99b 100644
--- a/core/lib/Drupal/Core/Field/EntityReferenceFieldItemList.php
+++ b/core/lib/Drupal/Core/Field/EntityReferenceFieldItemList.php
@@ -2,6 +2,7 @@
 
 namespace Drupal\Core\Field;
 
+use Drupal\Core\Entity\ContentEntityInterface;
 use Drupal\Core\Entity\FieldableEntityInterface;
 use Drupal\Core\Form\FormStateInterface;
 
@@ -17,7 +18,7 @@ public function getSerializationValue($deep_serialization) {
     if ($deep_serialization) {
       $values = $this->getValue(TRUE);
       foreach ($values as $delta => $item_values) {
-        if (isset($values[$delta]['entity']) && ($values[$delta]['entity'] instanceof FieldableEntityInterface)) {
+        if (isset($values[$delta]['entity']) && ($values[$delta]['entity'] instanceof ContentEntityInterface)) {
           $values[$delta]['entity']->setDeepSerialization(TRUE);
         }
       }