msg.sender.transfer(purchaseExcess); falla en la prueba de solidez

La transacción está fallando porque el contrato de prueba no tiene una función de reserva pagadera para aceptar la "transferencia" de fondos en exceso .

pragma solidity ^0.4.17;

import "../contracts/EmojiCoin.sol"; 
import "truffle/Assert.sol";
import "truffle/DeployedAddresses.sol";

contract TestEmojiCoin {
    uint public initialBalance = 1 ether;

    function testPurchase() public {
        address contractAddress = DeployedAddresses.EmojiCoin();
        EmojiCoin emojiCoin = EmojiCoin(contractAddress);

        // Emoji 0 is created in previous test.

        address owner_0 = emojiCoin.tokenOwner(0);
        Assert.notEqual( owner_0, this, "owner for coin 0 is incorrect" );

        emojiCoin.purchase.value(1 ether).gas(30000000000)(0);

        owner_0 = emojiCoin.tokenOwner(0);
        Assert.equal( owner_0, this, "owner for coin 0 is incorrect" );
    }

    // this new function IS REQUIRED for the test to work
    function() public payable { }

}

Se agregaron dos pruebas de afirmación (algunos podrían argumentar que esta no es la mejor práctica al tener más de una afirmación en la misma prueba)

Además, se tuvo que implementar un contrato de EmojiCoin muy básico para comprobar que este era el problema:

pragma solidity ^0.4.17;

import "zeppelin-solidity/contracts/math/SafeMath.sol";

contract EmojiCoin {

    mapping(uint256 => address) public tokenOwner;
    mapping(uint256 => uint256) public emojiIndexToPrice;
    uint256 public ownerCut  = 5;


    function EmojiCoin() public {

        // very simple constructor for the purpose of testing only
        tokenOwner[0] = msg.sender;
        emojiIndexToPrice[0] = 1 ether;

    }

    // simple _transfer implementation for the purpose of testing only
    function _transfer(address oldOwner, address newOwner, uint256 _tokenId) internal {
        require(tokenOwner[_tokenId] == oldOwner);
        tokenOwner[_tokenId] = newOwner;
    }

    function purchase(uint256 _tokenId) public payable {
        address oldOwner = tokenOwner[_tokenId];
        uint256 sellingPrice = emojiIndexToPrice[_tokenId];
        address newOwner = msg.sender;

        require(oldOwner != newOwner);

        require(newOwner != address(0));

        require(msg.value >= sellingPrice);

        uint256 percentage = SafeMath.sub(100, ownerCut);
        uint256 payment = uint256(SafeMath.div(SafeMath.mul(sellingPrice, percentage), 100));
        uint256 purchaseExcess = SafeMath.sub(msg.value, sellingPrice);

        emojiIndexToPrice[_tokenId] = SafeMath.div(SafeMath.mul(sellingPrice, 150), percentage);

        _transfer(oldOwner, newOwner, _tokenId);

        if (oldOwner != address(this)) {
           oldOwner.transfer(payment);
        }

        msg.sender.transfer(purchaseExcess);
    }


}