An error generated from Alphalens

I was learning alphalens by this website (here).
Although I've followed each lines of the tutorial, there is always error message in the last step.
Can anybody check the code for me? Thanks.

1
It looks like you're just missing a field argument in get_pricing. Try changing that line to:
pricing = get_pricing(assets, start_date = '2014-12-1', end_date = '2016-2-1', fields='open_price')