sql-sync fails on d6 installation.

When running sql-sync on an d6 installation brings the following stacktrace:

drush sql-sync -y @live @test
# ...
Do you really want to continue? (y/n): y
include_once(./includes/database/database.inc): failed to open stream: No such file or directory sql.drush.inc:657     [warning]
include_once(): Failed opening './includes/database/database.inc' for inclusion                                        [warning]
(include_path='.:/usr/share/php:/usr/share/pear') sql.drush.inc:657

IDrush expects a d7 (d6 has file "includes/database.php", d7 has "includes/database/database.inc") installation and fails.