Drush error: "Unknown engine type Database backend(s) options." [#1812714]

Running the following command breaks drush:

drush dl drubuntu

All subsquent drush commands fail with the following error message:

Unknown engine type Database backend(s) options.                                                   [error]
Unknown engine type Database backend(s) options.                                                   [error]

Fatal error: Call to undefined function conf_path() in ~php/drush/includes/bootstrap.inc on line 778
Drush command terminated abnormally due to an unrecoverable error.                                 [error]
Error: Call to undefined function conf_path() in ~php/drush/includes/bootstrap.inc,
line 778

The fix: