diff --git a/core/tests/Drupal/Tests/Core/Datetime/DateHelperTest.php b/core/tests/Drupal/Tests/Core/Datetime/DateHelperTest.php
index 991a02b..a88861c 100644
--- a/core/tests/Drupal/Tests/Core/Datetime/DateHelperTest.php
+++ b/core/tests/Drupal/Tests/Core/Datetime/DateHelperTest.php
@@ -28,6 +28,7 @@ public function testWeekDaysOrdered($first_day, $expected) {
     \Drupal::setContainer($container);
 
     $weekdays = DateHelper::weekDaysUntranslated();
+    // self::assertSame() MUST be used here as it checks for array key order.
     $this->assertSame($expected, DateHelper::weekDaysOrdered($weekdays));
   }