diff --git a/core/modules/migrate_drupal_ui/src/Tests/MigrateUpgradeTestBase.php b/core/modules/migrate_drupal_ui/src/Tests/MigrateUpgradeTestBase.php
index af91b4d..583dd10 100644
--- a/core/modules/migrate_drupal_ui/src/Tests/MigrateUpgradeTestBase.php
+++ b/core/modules/migrate_drupal_ui/src/Tests/MigrateUpgradeTestBase.php
@@ -161,6 +161,9 @@ protected function testMigrateUpgrade() {
     foreach ($all_migrations as $migration) {
       $id_map = $migration->getIdMap();
       foreach ($id_map as $source_id => $map) {
+        // Convert $source_id into a keyless array so that
+        // \Drupal\migrate\Plugin\migrate\id_map\Sql::getSourceHash() works as
+        // expected.
         $source_id_values = array_values(unserialize($source_id));
         $row = $id_map->getRowBySource($source_id_values);
         $destination = serialize($id_map->currentDestination());