diff --git a/core/tests/Drupal/Tests/Core/Form/FormCacheTest.php b/core/tests/Drupal/Tests/Core/Form/FormCacheTest.php
index 452ac54..e846b8d 100644
--- a/core/tests/Drupal/Tests/Core/Form/FormCacheTest.php
+++ b/core/tests/Drupal/Tests/Core/Form/FormCacheTest.php
@@ -427,6 +427,8 @@ public function testSetCacheAuthUser() {
    * @covers ::setCache
    */
   public function testSetCacheWithSafeStrings() {
+    // A call to SafeMarkup::set() is appropriate in this test as a way to add a
+    // string to the safe list in the simplest way possible. Normally, avoid it.
     SafeMarkup::set('a_safe_string');
     $form_build_id = 'the_form_build_id';
     $form = [