Logging error when making backups with 'drush bb' command

[Wylbur]

When making backups today, the following error was generated when creating a backup :

Error: Class 'Drupal\Core\Logger\RfcLogLevel' not found in drush_watchdog_severity_levels() (line 294 of /var/www/drush/commands/core/drupal/environment_common.inc).

In using drupal drush project 8.4.9-dev, and latest backdrop drush.

When this happens, the log message about the backup creating is also not created.
The backup file is successful however.