During the algorithm initialization, the following exception has occurred: Please register to receive data for symbol ' ' using the AddSecurity() function. 

private const string _symbol = "EURUSD";

// properties
ExponentialMovingAverage _emaFast;
ExponentialMovingAverage _emaSlow;

public override void Initialize()
{
// Backtest only
SetStartDate(2017, 5, 1);
SetEndDate(2017, 5, 31);
SetCash(100000);

_emaFast = EMA(_symbol, 20);
_emaSlow = EMA(_symbol, 40);

// Find more symbols here: http://quantconnect.com/data
AddForex(_symbol, Resolution.Minute, Market.Oanda);

AddSecurity(SecurityType.Forex, _symbol, Resolution.Minute);

Securities[_symbol].SetDataNormalizationMode(DataNormalizationMode.Raw);

SetBrokerageModel(BrokerageName.OandaBrokerage);
}