.../src/Plugin/rest/resource/EntityResource.php    | 26 ++++++++++------------
 core/modules/rest/src/Tests/UpdateTest.php         |  3 ++-
 2 files changed, 14 insertions(+), 15 deletions(-)

diff --git a/core/modules/rest/src/Plugin/rest/resource/EntityResource.php b/core/modules/rest/src/Plugin/rest/resource/EntityResource.php
index 18a1101..be66ee8 100644
--- a/core/modules/rest/src/Plugin/rest/resource/EntityResource.php
+++ b/core/modules/rest/src/Plugin/rest/resource/EntityResource.php
@@ -4,6 +4,7 @@
 
 use Drupal\Component\Plugin\DependentPluginInterface;
 use Drupal\Core\Config\Entity\ConfigEntityType;
+use Drupal\Core\Entity\ContentEntityInterface;
 use Drupal\Core\Entity\EntityTypeManagerInterface;
 use Drupal\Core\Entity\FieldableEntityInterface;
 use Drupal\Core\Config\ConfigFactoryInterface;
@@ -213,20 +214,17 @@ public function patch(EntityInterface $original_entity, EntityInterface $entity
     foreach ($entity->_restSubmittedFields as $field_name) {
       $field = $entity->get($field_name);
 
-      // Entity key fields need special treatment: together they uniquely
-      // identify the entity. Therefore it does not make sense to modify any of
-      // them. However, rather than throwing an error, we just ignore them as
-      // long as their specified values match their current values.
-      if (in_array($field_name, $entity_keys, TRUE)) {
-        // Unchanged values for entity keys don't need access checking.
-        if ($original_entity->get($field_name)->getValue() === $entity->get($field_name)->getValue()) {
-          continue;
-        }
-        // It is not possible to set the language to NULL as it is automatically
-        // re-initialized. As it must not be empty, skip it if it is.
-        elseif (isset($entity_keys['langcode']) && $field_name === $entity_keys['langcode'] && $field->isEmpty()) {
-          continue;
-        }
+      // It is not possible to set the language to NULL as it is automatically
+      // re-initialized. As it must not be empty, skip it if it is.
+      if (isset($entity_keys['langcode']) && $field_name === $entity_keys['langcode'] && $field->isEmpty()) {
+        continue;
+      }
+
+      // Allow sending read-only fields, as long as their value is unchanged.
+      if ($entity instanceof ContentEntityInterface
+        && $field->getFieldDefinition()->isReadOnly()
+        && $original_entity->get($field_name)->getValue() === $entity->get($field_name)->getValue()) {
+        continue;
       }
 
       if (!$original_entity->get($field_name)->access('edit')) {
diff --git a/core/modules/rest/src/Tests/UpdateTest.php b/core/modules/rest/src/Tests/UpdateTest.php
index cdef82c..727324f 100644
--- a/core/modules/rest/src/Tests/UpdateTest.php
+++ b/core/modules/rest/src/Tests/UpdateTest.php
@@ -283,6 +283,7 @@ public function testUpdateComment() {
     $entity_values['uid'] = $account->id();
     $comment = Comment::create($entity_values);
     $comment->save();
+    $comment_id = $comment->id();
 
     $this->pass('Test case 1: PATCH comment using HAL+JSON.');
     $comment->setSubject('Initial subject')->save();
@@ -300,7 +301,7 @@ 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');
-    $comment = Comment::load($comment->id());
+    $comment = Comment::load($comment_id);
     $this->assertEqual('Updated subject', $comment->getSubject());
 
     $this->pass('Test case 1: PATCH comment using JSON.');