diff --git a/core/modules/field/src/Plugin/migrate/process/d7/FieldInstanceDefaults.php b/core/modules/field/src/Plugin/migrate/process/d7/FieldInstanceDefaults.php
index a6707bd..b9d9bc0 100644
--- a/core/modules/field/src/Plugin/migrate/process/d7/FieldInstanceDefaults.php
+++ b/core/modules/field/src/Plugin/migrate/process/d7/FieldInstanceDefaults.php
@@ -20,6 +20,9 @@ public function transform($value, MigrateExecutableInterface $migrate_executable
     list($default_value, $widget_settings) = $value;
     $widget_type = $widget_settings['type'];
     $default_value = $default_value ?: [];
+
+    // In Drupal 7, the default value for email fields is stored in the key
+    // 'email' while in Drupal 8 it is stored in the key 'value'.
     if ($widget_type == 'email_textfield' && $default_value) {
       $default_value[0]['value'] = $default_value[0]['email'];
       unset($default_value[0]['email']);