PHP error when non-admin user edits program with no DJs

My log had several entries like this:

Invalid argument supplied for foreach() in /home/seasider/public_html/sites/all/modules/station/schedule/station_schedule.module on line 285.

These appear to be caused by a non-admin user editing a program.
He has the right to, because I set him as author of the node.
However, the node has no DJs set yet, because I've not got all the presenters set up with user accounts.

The problem is in the hook_nodeapi implementation:

        // validate the users
        foreach ($node->djs as $key => $dj) {
          if ($dj['username']) {
            $user = user_load(array('name' => $dj['username']));
            if (!$user->uid) {
              form_set_error("djs][$key][username", t('You need to specify a valid user name.'));
            }
            if (!isset($user->roles[$dj_role])) {
              form_set_error("djs][$key][username", t('%username is not a member of the %rolename role.',
                  array('%username' => $user->name, '%rolename' => $roles[$dj_role])));
            }
          }
        }

Looks like we need a test for $node->djs at the top of that case.