diff -u b/core/lib/Drupal/Core/Field/EntityReferenceFieldItemList.php b/core/lib/Drupal/Core/Field/EntityReferenceFieldItemList.php
--- b/core/lib/Drupal/Core/Field/EntityReferenceFieldItemList.php
+++ b/core/lib/Drupal/Core/Field/EntityReferenceFieldItemList.php
@@ -17,7 +17,7 @@
     if ($deep_serialization) {
       $values = $this->getValue(TRUE);
       foreach ($values as $delta => $item_values) {
-        if (isset($values[$delta]['entity'])) {
+        if (isset($values[$delta]['entity']) && ($values[$delta]['entity'] instanceof FieldableEntityInterface)) {
           $values[$delta]['entity']->setDeepSerialization();
         }
       }