I'm trying to get started with Backtrader and I want to use YahooFinanceData to download some data. However, it seems to not be downloading anything. In my strategy, len(self.datas[0]) is equal to zero. I've pasted the full code below. My code is nearly identical to the Quickstart, but with YahooFinanceData instead of YahooFinanceCSVData. Any ideas?

Thanks,

from __future__ import (absolute_import, division, print_function, unicode_literals)
import backtrader as bt
import datetime

class PrintPrices(bt.Strategy):

    def log(self, txt, dt=None):
        dt = dt or self.datas[0].datetime.date(0)
        print('%s, %s' % (dt.isoformat(), txt))

    def __init__(self):
        self.dataclose = self.datas[0].close
        print('%d' % len(self.datas[0]))

    def next(self):
        self.log('Close, %.2f' % self.dataclose[0])


if __name__ == '__main__':
    cerebro = bt.Cerebro()
    cerebro.addstrategy(PrintPrices)
    cerebro.broker.setcash(100000.0)

    voo_data = bt.feeds.YahooFinanceData(
        dataname = 'VOO',
        name = 'VOO',
        fromdate = datetime.datetime(2015,1,1),
        todate = datetime.datetime(2017,3,1),
        reverse = False
    )

    cerebro.adddata(voo_data)
    cerebro.run()