diff --git a/core/modules/hal/src/Normalizer/FieldItemNormalizer.php b/core/modules/hal/src/Normalizer/FieldItemNormalizer.php
index 5ad472a..7ff11dd 100644
--- a/core/modules/hal/src/Normalizer/FieldItemNormalizer.php
+++ b/core/modules/hal/src/Normalizer/FieldItemNormalizer.php
@@ -22,8 +22,10 @@ class FieldItemNormalizer extends NormalizerBase {
    */
   public function normalize($field_item, $format = NULL, array $context = array()) {
     $values = [];
+    /** @var \Drupal\Core\TypedData\TypedDataInterface $data_item */
     foreach ($field_item as $property => $data_item) {
-      $values[$property] = $this->serializer->normalize($data_item, $format, $context);
+      // @todo Support nullable types on the lowest levels.
+      $values[$property] = $data_item->getValue() === NULL ? NULL : $this->serializer->normalize($data_item, $format, $context);
     }
 
     if (isset($context['langcode'])) {
diff --git a/core/modules/rest/src/Tests/PageCacheTest.php b/core/modules/rest/src/Tests/PageCacheTest.php
index 66e57f5..37e7273 100644
--- a/core/modules/rest/src/Tests/PageCacheTest.php
+++ b/core/modules/rest/src/Tests/PageCacheTest.php
@@ -56,6 +56,7 @@ public function testConfigChangePageCache() {
     $entity = $this->entityCreate('entity_test');
     $entity->set('field_test_text', 'custom cache tag value');
     $serialized = $this->serializer->serialize($entity, $this->defaultFormat);
+
     // Log in for creating the entity.
     $this->drupalLogin($account);
     $this->httpRequest('entity/entity_test', 'POST', $serialized, $this->defaultMimeType);
diff --git a/core/modules/rest/src/Tests/UpdateTest.php b/core/modules/rest/src/Tests/UpdateTest.php
index 287c2df..20397ce 100644
--- a/core/modules/rest/src/Tests/UpdateTest.php
+++ b/core/modules/rest/src/Tests/UpdateTest.php
@@ -292,6 +292,7 @@ public function testUpdateComment() {
     $this->pass('Test case 1: PATCH comment using HAL+JSON.');
     $comment->setSubject('Initial subject')->save();
     $read_only_fields = [
+      'cid',
       'status',
       'name',
       'created',
@@ -304,13 +305,14 @@ public function testUpdateComment() {
     ];
     $this->assertNotEqual('Updated subject', $comment->getSubject());
     $comment->setSubject('Updated subject');
-    $this->patchEntity($comment, $read_only_fields, $account, 'hal_json', 'application/hal+json');
+    $this->patchEntity(clone $comment, $read_only_fields, $account, 'hal_json', 'application/hal+json');
     $comment = Comment::load($comment->id());
     $this->assertEqual('Updated subject', $comment->getSubject());
 
     $this->pass('Test case 1: PATCH comment using JSON.');
     $comment->setSubject('Initial subject')->save();
     $read_only_fields = [
+      'cid',
       'status',
       'pid', // Extra compared to HAL+JSON.
       'entity_id',
@@ -325,7 +327,7 @@ public function testUpdateComment() {
     ];
     $this->assertNotEqual('Updated subject', $comment->getSubject());
     $comment->setSubject('Updated subject');
-    $this->patchEntity($comment, $read_only_fields, $account, 'json', 'application/json');
+    $this->patchEntity(clone $comment, $read_only_fields, $account, 'json', 'application/json');
     $comment = Comment::load($comment->id());
     $this->assertEqual('Updated subject', $comment->getSubject());
   }
diff --git a/core/modules/serialization/src/Normalizer/ComplexDataNormalizer.php b/core/modules/serialization/src/Normalizer/ComplexDataNormalizer.php
index 5062738..fd170dc 100644
--- a/core/modules/serialization/src/Normalizer/ComplexDataNormalizer.php
+++ b/core/modules/serialization/src/Normalizer/ComplexDataNormalizer.php
@@ -26,8 +26,10 @@ class ComplexDataNormalizer extends NormalizerBase {
    */
   public function normalize($object, $format = NULL, array $context = array()) {
     $attributes = array();
+    /** @var \Drupal\Core\TypedData\TypedDataInterface $field */
     foreach ($object as $name => $field) {
-      $attributes[$name] = $this->serializer->normalize($field, $format, $context);
+      // @todo Support nullable types on the lowest levels.
+      $attributes[$name] = $field->getValue() === NULL ? NULL : $this->serializer->normalize($field, $format, $context);
     }
     return $attributes;
   }