Update to Drupal 8.0.0-beta15. For more information, see: https://www.drupal.org/node/2563023

core/modules/serialization/serialization.services.yml

@@ -30,6 +30,10 @@ services:
       arguments: ['Drupal\Core\Field\Plugin\Field\FieldType\PasswordItem']
       tags:
         - { name: normalizer, priority: 20 }
+  serializer.normalizer.safe_string:
+      class: Drupal\serialization\Normalizer\SafeStringNormalizer
+      tags:
+        - { name: normalizer }
   serializer.normalizer.typed_data:
     class: Drupal\serialization\Normalizer\TypedDataNormalizer
     tags:

core/modules/serialization/src/Normalizer/SafeStringNormalizer.php

@@ -0,0 +1,29 @@
+<?php
+
+/**
+ * @file
+ * Contains \Drupal\serialization\Normalizer\SafeStringNormalizer.
+ */
+
+namespace Drupal\serialization\Normalizer;
+
+/**
+ * Normalizes SafeStringInterface objects into a string.
+ */
+class SafeStringNormalizer extends NormalizerBase {
+
+  /**
+   * The interface or class that this Normalizer supports.
+   *
+   * @var array
+   */
+  protected $supportedInterfaceOrClass = array('Drupal\Component\Utility\SafeStringInterface');
+
+  /**
+   * {@inheritdoc}
+   */
+  public function normalize($object, $format = NULL, array $context = array()) {
+    return (string) $object;
+  }
+
+}