diff -u b/core/modules/migrate_drupal_ui/src/Tests/MigrateUpgradeTestBase.php b/core/modules/migrate_drupal_ui/src/Tests/MigrateUpgradeTestBase.php
--- b/core/modules/migrate_drupal_ui/src/Tests/MigrateUpgradeTestBase.php
+++ b/core/modules/migrate_drupal_ui/src/Tests/MigrateUpgradeTestBase.php
@@ -118,12 +118,15 @@
     $drivers = drupal_get_database_types();
     $form = $drivers[$driver]->getFormOptions($connection_options);
     $connection_options = array_intersect_key($connection_options, $form + $form['advanced_options']);
+    $version = $this->getSourceDrupalVersion();
     $edit = [
       $driver => $connection_options,
-      'source_base_path' => $this->getSourceBasePath(),
       'source_private_file_path' => $this->getSourcePrivateFilePath(),
-      'version' => $this->getSourceDrupalVersion(),
+      'version' => $version,
     ];
+    if ($version == 6) {
+      $edit['d6_source_base_path'] = $this->getSourceBasePath();
+    }
     if (count($drivers) !== 1) {
       $edit['driver'] = $driver;
     }