.../ConfigurableLanguageResourceTestBase.php           | 18 ++++++++++++++++++
 .../src/Normalizer/ConfigEntityNormalizer.php          |  2 +-
 .../src/Unit/Normalizer/ConfigEntityNormalizerTest.php | 10 ++++++++--
 3 files changed, 27 insertions(+), 3 deletions(-)

diff --git a/core/modules/rest/tests/src/Functional/EntityResource/ConfigurableLanguage/ConfigurableLanguageResourceTestBase.php b/core/modules/rest/tests/src/Functional/EntityResource/ConfigurableLanguage/ConfigurableLanguageResourceTestBase.php
index 301beb2..cf5be74 100644
--- a/core/modules/rest/tests/src/Functional/EntityResource/ConfigurableLanguage/ConfigurableLanguageResourceTestBase.php
+++ b/core/modules/rest/tests/src/Functional/EntityResource/ConfigurableLanguage/ConfigurableLanguageResourceTestBase.php
@@ -3,6 +3,7 @@
 namespace Drupal\Tests\rest\Functional\EntityResource\ConfigurableLanguage;
 
 use Drupal\Core\Cache\Cache;
+use Drupal\Core\Url;
 use Drupal\Tests\rest\Functional\EntityResource\EntityResourceTestBase;
 use Drupal\language\Entity\ConfigurableLanguage;
 
@@ -74,4 +75,21 @@ protected function getNormalizedPostEntity() {
     // @todo Update in https://www.drupal.org/node/2300677.
   }
 
+  /**
+   * Test a GET request for a default config entity, which has a _core key.
+   *
+   * @see https://www.drupal.org/node/2915414
+   */
+  public function testGetDefaultConfig() {
+    $this->initAuthentication();
+    $url = Url::fromUri('base:/entity/configurable_language/en')->setOption('query', ['_format' => static::$format]);;
+    $request_options = $this->getAuthenticationRequestOptions('GET');
+    $this->provisionEntityResource();
+    $this->setUpAuthorization('GET');
+    $response = $this->request('GET', $url, $request_options);
+
+    $normalization = $this->serializer->decode((string) $response->getBody(), static::$format);
+    $this->assertArrayNotHasKey('_core', $normalization);
+  }
+
 }
diff --git a/core/modules/serialization/src/Normalizer/ConfigEntityNormalizer.php b/core/modules/serialization/src/Normalizer/ConfigEntityNormalizer.php
index 344bcc3..dd766ce 100644
--- a/core/modules/serialization/src/Normalizer/ConfigEntityNormalizer.php
+++ b/core/modules/serialization/src/Normalizer/ConfigEntityNormalizer.php
@@ -18,7 +18,7 @@ class ConfigEntityNormalizer extends EntityNormalizer {
    * {@inheritdoc}
    */
   public function normalize($object, $format = NULL, array $context = []) {
-    return $object->toArray();
+    return array_diff_key($object->toArray(), ['_core' => TRUE]);
   }
 
 }
diff --git a/core/modules/serialization/tests/src/Unit/Normalizer/ConfigEntityNormalizerTest.php b/core/modules/serialization/tests/src/Unit/Normalizer/ConfigEntityNormalizerTest.php
index 371450a..2f99e2d 100644
--- a/core/modules/serialization/tests/src/Unit/Normalizer/ConfigEntityNormalizerTest.php
+++ b/core/modules/serialization/tests/src/Unit/Normalizer/ConfigEntityNormalizerTest.php
@@ -17,7 +17,13 @@ class ConfigEntityNormalizerTest extends UnitTestCase {
    * @covers ::normalize
    */
   public function testNormalize() {
-    $test_export_properties = ['test' => 'test'];
+    $test_export_properties = [
+      'test' => 'test',
+      '_core' => [
+        'default_config_hash' => $this->randomMachineName(),
+        $this->randomMachineName() => 'some random key',
+      ],
+    ];
 
     $entity_manager = $this->getMock('Drupal\Core\Entity\EntityManagerInterface');
     $normalizer = new ConfigEntityNormalizer($entity_manager);
@@ -27,7 +33,7 @@ public function testNormalize() {
       ->method('toArray')
       ->will($this->returnValue($test_export_properties));
 
-    $this->assertSame($test_export_properties, $normalizer->normalize($config_entity));
+    $this->assertSame(['test' => 'test'], $normalizer->normalize($config_entity));
   }
 
 }