在js VM环境中没问题，部署到本地私链后，测试转账会报这个错误
//转账方法
function transfer(address _to, uint256 _amount) public returns (bool success) {
require(
balanceMap[_to] + _amount >= balanceMap[_to],
"check for overflows"
);
require(balanceMap[msg.sender] >= _amount, "check money");
balanceMap[msg.sender] -= _amount;
balanceMap[_to] += _amount;
emit Transfer(msg.sender, _to, _amount);
return true;
}
