function _deliverBonusTokens(address beneficiary)
    private
  {
    require(
      _lockedTokens[beneficiary] > 0,
      "No tokens to unlock."
    );

    _lockedTokens[beneficiary] = 0;

    require(
      ERC20(address(token())).transfer(beneficiary, _lockedTokens[beneficiary]),
      "Could not transfer tokens."
    );
  }