UID is null when saving anonymous user

When adding a new user anonymously, CM fires hook_campaignmonitor_subscribe(), which is picked up by:

function campaignmonitor_local_campaignmonitor_subscribe($list_id, $email) {
  // Add local record.
  $uid = campaignmonitor_get_uid_from_email($email);
  campaignmonitor_local_dbmerge($uid, $list_id);
}

However, $uid will come back null and the operation will fail. This is my workaround but isn't likely to be correct:

function campaignmonitor_local_campaignmonitor_subscribe($list_id, $email) {
  // Add local record.
  $uid = campaignmonitor_get_uid_from_email($email);
  if (isset($uid) && is_numeric($uid)) {
    campaignmonitor_local_dbmerge($uid, $list_id);
  }
}

For the moment it prevents the user from being told the subscription failed (it actually has updated remotely at this point).