'exec' doesn't work with group alias

I have a site alias @foo for a remote site accessed over ssh. This works as expected:

drush @foo exec ls

I also have a site alias @bar for another remote site, and a site alias that groups them both together:

// Group alias.
$aliases['both'] = array(
  'site-list' => array('@foo', '@bar'),
);

However, trying to exec a command on both sites with this doesn't work:

drush @both exec ls

This appears to just run the ls command locally.