var symbols = ["BTC_USDT", "LTC_USDT", "EOS_USDT", "ETH_USDT", "BCC_USDT"];
var buyValue = 1000;
function main(){
for(var i=0;i<symbols.length;i++){
exchange.IO("currency", symbols[i]);
var ticker = exchange.GetTicker();
var amount = _N(buyValue/ticker.Sell, 3);
exchange.Buy(ticker.Sell, amount);
Sleep(1000);
}
}

我用bitfinex BTC_USDT交易平台做模拟回测，就看到这个error。上面代码是Exchange的API说明书里的：https://fmz-docs.readthedocs.io/en/latest/code_Instruction/Extent API.html#io
怎么破？
main:5:18 - TypeError: Cannot convert "BTC_USDT" to int