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
@@ -121,6 +121,7 @@
     $version = $this->getSourceDrupalVersion();
     $edit = [
       $driver => $connection_options,
+      'source_base_path' => $this->getSourceBasePath(),
       'source_private_file_path' => $this->getSourcePrivateFilePath(),
       'version' => $version,
     ];