diff -u b/core/modules/rest/tests/src/Functional/EntityResource/EntityResourceTestBase.php b/core/modules/rest/tests/src/Functional/EntityResource/EntityResourceTestBase.php
--- b/core/modules/rest/tests/src/Functional/EntityResource/EntityResourceTestBase.php
+++ b/core/modules/rest/tests/src/Functional/EntityResource/EntityResourceTestBase.php
@@ -696,7 +696,7 @@
 
     // DX: 422 when invalid entity: UUID field too long.
     // @todo Fix this in https://www.drupal.org/node/2149851.
-    if ($this->entity->uuid()) {
+    if ($this->entity->getEntityType()->hasKey('uuid')) {
       $response = $this->request('POST', $url, $request_options);
       $this->assertResourceErrorResponse(422, "Unprocessable Entity: validation failed.\nuuid.0.value: UUID: may not be longer than 128 characters.\n", $response);
     }
diff -u b/core/modules/rest/tests/src/Functional/EntityResource/Item/ItemResourceTestBase.php b/core/modules/rest/tests/src/Functional/EntityResource/Item/ItemResourceTestBase.php
--- b/core/modules/rest/tests/src/Functional/EntityResource/Item/ItemResourceTestBase.php
+++ b/core/modules/rest/tests/src/Functional/EntityResource/Item/ItemResourceTestBase.php
@@ -55,13 +55,13 @@
    */
   protected function createEntity() {
     // Create a "Camelids" feed.
-    $feed = Feed::create(array(
+    $feed = Feed::create([
       'title' => 'Camelids',
       'url' => 'https://groups.drupal.org/not_used/167169',
       'refresh' => 900,
       'checked' => 1389919932,
       'description' => 'Drupal Core Group feed',
-    ));
+    ]);
     $feed->save();
 
     // Create a "Llama" item.
@@ -95,7 +95,7 @@
       ],
       'fid' => [
         [
-          'target_id' => (int) $feed->id(),
+          'target_id' => 1,
           'target_type' => 'aggregator_feed',
           'target_uuid' => $feed->uuid(),
           'url' => $feed->toUrl()->toString(),