function _preValidatePurchase(
    address _beneficiary,
    uint256 _weiAmount
  )
    internal
  {
    require(
      _weiAmount >= minimumContribution,
      "Can't send less than the minimum contribution"
    );

    
    if (contributions.ethContributions(_beneficiary).add(_weiAmount) > tierZero) {
      require(
        contributions.isAllowedPurchase(_beneficiary, _weiAmount),
        "Beneficiary is not allowed to purchase this amount"
      );
    }

    super._preValidatePurchase(_beneficiary, _weiAmount);
  }