site migration fails on head with site not found

Site migration broke. Here's what I get when trying to migrate a fresh 5.16 with no contrib modules to a 6.9 install:

Task starts processing
Task dispatcher running: php /var/hostmaster/drush/drush.php --root='/var/hostmaster/drupal-5.16' 'provision' 'migrate' '/var/hostmaster/drupal-6.9' 'testup56' --backend
Drush bootstrap phase : _drush_bootstrap_drush()
Drush bootstrap phase : _drush_bootstrap_drupal_root()
Loading drushrc "/var/hostmaster/drupal-5.16/drushrc.php" into "drupal" scope.
Initialized Drupal 5.16 root directory at /var/hostmaster/drupal-5.16
Initializing provision_drupal
Initializing provision_apache
Initializing provision_mysql
Drush bootstrap phase : _drush_bootstrap_drupal_site()
Initialized Drupal site at sites/
PROVISION_DRUPAL_SITE_FOUND
An error occurred at function : provision_drupal_provision_backup_validate
An error occurred at function : provision_drupal_provision_pre_migrate
Removing task from hosting queue

This is a separate issue from #408810: site migration fails on head.