update tries and fails to update themes

Beginning in Drupal 6.15, the update module reports on the update status of themes as well as modules. When a theme is out of date, update reports that, which is good.

Unfortunately, this confuses drush, which thinks it's trying to update a module and throws an error that it can't find the path. Here's what happens on my host when drush tries to update the pixture_reloaded theme:

$ drush update
Refreshing update status information ...
Done.
Update information last refreshed: Fri, 12/18/2009 - 2:50pm

Update status information on all installed and enabled Drupal modules:
 Name                            Installed    Proposed     Status
                                 version      version
 <snip>
 Pixture Reloaded                6.x-3.0      6.x-3.2      Update
                                                           available

Code updates will be made to the following projects:
Pixture Reloaded [pixture_reloaded-6.x-3.2]

Note: Updated modules can potentially break your site. It's not recommended to update production sites without prior testing.
Note: A backup of your package will be stored to backups directory if it is not managed by a supported version control system.
Note: If you have made any modifications to any file that belongs to one of these projects, you will have to migrate those modifications after updating.
Do you really want to continue? (y/n): y
Module pixture_reloaded path is not available, perhaps the module is [error]
enabled but has been deleted from disk.