diff --git a/core/modules/migrate_drupal/src/Plugin/migrate/source/d6/CckFieldValues.php b/core/modules/migrate_drupal/src/Plugin/migrate/source/d6/CckFieldValues.php
index c2f8739..87f131c 100644
--- a/core/modules/migrate_drupal/src/Plugin/migrate/source/d6/CckFieldValues.php
+++ b/core/modules/migrate_drupal/src/Plugin/migrate/source/d6/CckFieldValues.php
@@ -108,7 +108,8 @@ public function prepareRow(Row $row) {
       if (count($query->getFields())) {
         if ($results = $query->execute()->fetchAssoc()) {
           $source = $row->getSource();
-          // We diff the results because the extra will be all the field columns.
+          // We diff the results with the source to find any field columns
+          // in the content type's main table.
           $new_fields = array_diff($results, $source);
           foreach ($new_fields as $key => $value) {
             $row->setSourceProperty($key, $value);