Failing test

MyContract.sol

pragma solidity ^0.4.18;

contract MyContract {

  mapping(address => uint) totalTokens;

  function buy() payable public {
    totalTokens[msg.sender] = msg.value;
  }

  function tokenCount(address addr) public view returns (uint) {
    return totalTokens[addr];
  }
}

TestMyContract.sol

pragma solidity ^0.4.18;

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

contract TestMyContract {

  function testBuyTokens() public {
    MyContract c = MyContract(DeployedAddresses.MyContract());
    c.buy.value(1 ether)();
  }

}