drush pm update can't be executed

i can run like every drush command, but not the package manager ones

drush --verbose pm update
Initialized Drupal 6.10 root directory at /var/www/drupal                                                                       [notice]
Initialized Drupal site default at sites/default                                                                                [notice]
The command 'drush.php pm update' could not be executed.                                                                        [error]

i have
- ubuntu 8.10
- php: 5.2.6-2ubuntu4.2
- drupal 6.10

thank your for your advice,
josef