diff --git a/core/modules/rest/lib/Drupal/rest/Plugin/rest/resource/EntityResource.php b/core/modules/rest/lib/Drupal/rest/Plugin/rest/resource/EntityResource.php
index 42d0335..95a2587 100644
--- a/core/modules/rest/lib/Drupal/rest/Plugin/rest/resource/EntityResource.php
+++ b/core/modules/rest/lib/Drupal/rest/Plugin/rest/resource/EntityResource.php
@@ -102,13 +102,20 @@ public function put($id, EntityInterface $entity) {
     if (empty($id)) {
       throw new NotFoundHttpException();
     }
+    $definition = $this->getDefinition();
+    $original_entity = entity_load($definition['entity_type'], $id);
+    // We don't support creating entities with PUT, so we throw an error if
+    // there is no existing entity.
+    if ($original_entity == FALSE) {
+      throw new NotFoundHttpException();
+    }
     $info = $entity->entityInfo();
     // Make sure that the entity ID is the one provided in the URL.
     $entity->{$info['entity_keys']['id']} = $id;
     try {
       $entity->save();
       // Update responses have an empty body.
-      return new ResourceResponse(NULL, 200);
+      return new ResourceResponse(NULL, 204);
     }
     catch (EntityStorageException $e) {
       throw new HttpException(500, 'Internal Server Error', $e);
diff --git a/core/modules/rest/lib/Drupal/rest/Tests/UpdateTest.php b/core/modules/rest/lib/Drupal/rest/Tests/UpdateTest.php
index 1340d79..f47cd7e 100644
--- a/core/modules/rest/lib/Drupal/rest/Tests/UpdateTest.php
+++ b/core/modules/rest/lib/Drupal/rest/Tests/UpdateTest.php
@@ -58,7 +58,7 @@ public function testUpdate() {
     $serialized = $serializer->serialize($update_entity, 'drupal_jsonld');
     // Update the entity over the web API.
     $this->httpRequest('entity/' . $entity_type . '/' . $entity->id(), 'PUT', $serialized, 'application/vnd.drupal.ld+json');
-    $this->assertResponse(200);
+    $this->assertResponse(204);
 
     // Re-load the updated entity from the database.
     $entity = entity_load($entity_type, $entity->id(), TRUE);
@@ -75,12 +75,18 @@ public function testUpdate() {
     unset($update_entity->field_test_text);
     $serialized = $serializer->serialize($update_entity, 'drupal_jsonld');
     $this->httpRequest('entity/' . $entity_type . '/' . $entity->id(), 'PUT', $serialized, 'application/vnd.drupal.ld+json');
-    $this->assertResponse(200);
+    $this->assertResponse(204);
 
     // Re-load the updated entity from the database.
     $entity = entity_load($entity_type, $entity->id(), TRUE);
     $this->assertTrue($entity->field_test_text->isEmpty(), 'Property has been deleted.');
 
+    // Try to create an entity with ID 9999.
+    $this->httpRequest('entity/' . $entity_type . '/9999', 'PUT', $serialized, 'application/vnd.drupal.ld+json');
+    $this->assertResponse(404);
+    $loaded_entity = entity_load($entity_type, 9999, TRUE);
+    $this->assertFalse($loaded_entity, 'Entity 9999 was not created.');
+
     // Try to update an entity without proper permissions.
     $this->drupalLogout();
     $this->httpRequest('entity/' . $entity_type . '/' . $entity->id(), 'PUT', $serialized, 'application/vnd.drupal.ld+json');