PHP Fatal error: Call to undefined function drush_backend_invoke()

Any suggestions what might be causing this?

→ drush @hostmaster hosting-queue-runner
PHP Fatal error:  Call to undefined function drush_backend_invoke() in /private/var/aegir/hostmaster-6.x-2.x-dev/sites/aegir.ld/modules/hosting_queue_runner/hosting_queue_runner.drush.inc on line 96
PHP Stack trace:
PHP   1. {main}() /usr/local/Cellar/drush/5.4/libexec/drush.php:0
PHP   2. drush_main() /usr/local/Cellar/drush/5.4/libexec/drush.php:14
PHP   3. _drush_bootstrap_and_dispatch() /usr/local/Cellar/drush/5.4/libexec/drush.php:59
PHP   4. drush_dispatch() /usr/local/Cellar/drush/5.4/libexec/drush.php:90
PHP   5. call_user_func_array() /usr/local/Cellar/drush/5.4/libexec/includes/command.inc:165
PHP   6. drush_command() /usr/local/Cellar/drush/5.4/libexec/includes/command.inc:165
PHP   7. _drush_invoke_hooks() /usr/local/Cellar/drush/5.4/libexec/includes/command.inc:198
PHP   8. call_user_func_array() /usr/local/Cellar/drush/5.4/libexec/includes/command.inc:324
PHP   9. drush_hosting_queue_runner() /usr/local/Cellar/drush/5.4/libexec/includes/command.inc:324

Fatal error: Call to undefined function drush_backend_invoke() in /private/var/aegir/hostmaster-6.x-2.x-dev/sites/aegir.ld/modules/hosting_queue_runner/hosting_queue_runner.drush.inc on line 96

Call Stack:
    0.0003     693744   1. {main}() /usr/local/Cellar/drush/5.4/libexec/drush.php:0
    0.0129    4702248   2. drush_main() /usr/local/Cellar/drush/5.4/libexec/drush.php:14
    0.0662   12822440   3. _drush_bootstrap_and_dispatch() /usr/local/Cellar/drush/5.4/libexec/drush.php:59
    0.1768   27233760   4. drush_dispatch() /usr/local/Cellar/drush/5.4/libexec/drush.php:90
    0.2295   27244376   5. call_user_func_array() /usr/local/Cellar/drush/5.4/libexec/includes/command.inc:165
    0.2295   27244624   6. drush_command() /usr/local/Cellar/drush/5.4/libexec/includes/command.inc:165
    0.6160   27680328   7. _drush_invoke_hooks() /usr/local/Cellar/drush/5.4/libexec/includes/command.inc:198
    0.6167   27750312   8. call_user_func_array() /usr/local/Cellar/drush/5.4/libexec/includes/command.inc:324
    0.6167   27750560   9. drush_hosting_queue_runner() /usr/local/Cellar/drush/5.4/libexec/includes/command.inc:324

Drush command terminated abnormally due to an unrecoverable error.                                                                                   [error]
Error: Call to undefined function drush_backend_invoke() in
/private/var/aegir/hostmaster-6.x-2.x-dev/sites/aegir.ld/modules/hosting_queue_runner/hosting_queue_runner.drush.inc, line 96