diff --git a/core/modules/system/lib/Drupal/system/Tests/Serialization/EntitySerializationTest.php b/core/modules/system/lib/Drupal/system/Tests/Serialization/EntitySerializationTest.php
index 7933998..89fa5ce 100644
--- a/core/modules/system/lib/Drupal/system/Tests/Serialization/EntitySerializationTest.php
+++ b/core/modules/system/lib/Drupal/system/Tests/Serialization/EntitySerializationTest.php
@@ -142,16 +142,12 @@ public function testSerialize() {
       'field_test_text' => '<field_test_text><value>' . $this->values['field_test_text']['value'] . '</value><format>' . $this->values['field_test_text']['format'] . '</format></field_test_text>',
     );
     // Sort it in the same order as normalised.
-    $expected = array_merge(array_flip(array_keys($normalized)), $expected);
+    $expected = array_merge($normalized, $expected);
     // Add header and footer.
     array_unshift($expected, '<?xml version="1.0"?>' . PHP_EOL . '<response>');
     $expected[] = '</response>' . PHP_EOL;
     // Reduced the array to a string.
-    $expected = array_reduce($expected, function($result, $item) {
-      $result .= $item;
-      return $result;
-    });
-
+    $expected = implode('', $expected);
     // Test 'xml'. The output should match that of Symfony's XmlEncoder.
     $actual = $serializer->serialize($this->entity, 'xml');
     $this->assertIdentical($actual, $expected);