provision backup fails if files have been uploaded

If I create a node and attach a file to it, then try to run 'provision backup' from either the cli or from the Aegir web UI the tar command fails because the newly uploaded file is owned by www-data.www-data.

Here is the files dir containing the newly uploaded load2.png:

aegir@www-qa01-cois:~$ ll platforms/dev/drupal-6.14/sites/kickoff-demo.qa01.localhost/files/
total 60
drwxrws--- 5 aegir    www-data  4096 Dec 22 10:01 ./
drwxr-xr-x 5 aegir    aegir     4096 Dec 16 11:37 ../
-rw-rw-r-- 1 www-data www-data    93 Dec 22 10:01 .htaccess
drwxrws--- 2 aegir    www-data  4096 Dec 16 11:37 images/
-rw------- 1 www-data www-data 33269 Dec 22 10:01 load2.png
drwxrws--- 2 aegir    www-data  4096 Dec 16 11:37 pictures/
drwxrws--- 2 aegir    www-data  4096 Dec 22 10:01 tmp/
aegir@www-qa01-cois:~$

The provision backup command with verbose enabled:

aegir@www-qa01-cois:~$ /var/aegir/drush/drush.php -v --root=/var/aegir/platforms/dev/drupal-6.14 provision backup kickoff-demo.qa01.localhost
Initialized Drupal 6.14 root directory at                               [notice]
/var/aegir/platforms/dev/drupal-6.14
Undefined index:  base_url                                              [notice]
Undefined index:  db_url                                                [notice]
Initialized Drupal site kickoff-demo.qa01.localhost at                  [notice]
sites/kickoff-demo.qa01.localhost
mysqldump --defaults-file=/dev/fd/3                                     [notice]
-rsites/kickoff-demo.qa01.localhost/database.sql site_548
Undefined variable: indent                                              [notice]
Executing: mysqldump --defaults-file=/dev/fd/3 -rsites/kickoff-demo.qa01.localhost/database.sql site_548
tar -C                                                                  [notice]
/var/aegir/platforms/dev/drupal-6.14/sites/kickoff-demo.qa01.localhost
-p -c -f
/var/aegir/backups/kickoff-demo.qa01.localhost-2009-12-22.tar .
Executing: tar -C /var/aegir/platforms/dev/drupal-6.14/sites/kickoff-demo.qa01.localhost -p -c -f /var/aegir/backups/kickoff-demo.qa01.localhost-2009-12-22.tar .
  tar: ./files/load2.png: Cannot open: Permission denied
  tar: Error exit delayed from previous errors
Could not back up sites directory for drupal                         [error]
An error occurred at function :                                      [error]
drush_provision_drupal_provision_backup
Deleted mysql dump from sites directory                                [message]
Command dispatch complete                                               [notice]