diff --git a/core/modules/migrate_drupal/lib/Drupal/migrate_drupal/Plugin/migrate/source/d6/CckFieldValues.php b/core/modules/migrate_drupal/lib/Drupal/migrate_drupal/Plugin/migrate/source/d6/CckFieldValues.php
index ba93e18..3c27a72 100644
--- a/core/modules/migrate_drupal/lib/Drupal/migrate_drupal/Plugin/migrate/source/d6/CckFieldValues.php
+++ b/core/modules/migrate_drupal/lib/Drupal/migrate_drupal/Plugin/migrate/source/d6/CckFieldValues.php
@@ -108,7 +108,7 @@ public function prepareRow(Row $row) {
       if ($results = $query->execute()->fetchAssoc()) {
         $source = $row->getSource();
         // We diff the results because the extra will be all the field columns.
-        $new_fields = array_diff($results, $source);
+        $new_fields = array_diff_key($results, $source);
         foreach ($new_fields as $key => $value) {
           $row->setSourceProperty($key, $value);
         }