diff --git a/core/lib/Drupal/Core/Render/Element/MachineName.php b/core/lib/Drupal/Core/Render/Element/MachineName.php
index 1c8b23b48a..762509f6d4 100644
--- a/core/lib/Drupal/Core/Render/Element/MachineName.php
+++ b/core/lib/Drupal/Core/Render/Element/MachineName.php
@@ -28,7 +28,8 @@
  *     - The form state object.
  *     In most cases, an existing API or menu argument loader function can be
  *     re-used. The callback is only invoked if the submitted value differs from
- *     the element's #default_value.
+ *     the element's initial #default_value. The initial #default_value is
+ *     stored in form state so AJAX forms can be reliably validated.
  *   - source: (optional) The #array_parents of the form element containing the
  *     human-readable name (i.e., as contained in the $form structure) to use as
  *     source for the machine name. Defaults to array('label').
@@ -260,7 +261,7 @@ public static function validateMachineName(&$element, FormStateInterface $form_s
 
     // Verify that the machine name is unique. If the value matches the initial
     // default value then it does not need to be validated as the machine name
-    // elements assumes the form is editing the existing value.
+    // element assumes the form is editing the existing value.
     $initial_values = $form_state->get('machine_name.initial_values') ?: [];
     $element_path = implode('][', $element['#parents']);
     if (!array_key_exists($element_path, $initial_values) || $initial_values[$element_path] !== $element['#value']) {