Invalid argument supplied for foreach()

When bringing up admin/settings/comment_notify, I receive:
warning: Invalid argument supplied for foreach() in /Applications/MAMP/htdocs/IGL-TGP/includes/form.inc on line 1193.
It looks like the checkbox default is being sent a boolean for comment_notify_available_alerts and it needs something else for the checkbox logic to work.

Line 1193 is the second one (from form.inc):

    foreach ($form['#default_value'] as $key) {
      $value[$key] = 1;
    }
    return $value;