포트폴리오 책보고 따라하는데 string indices must be integers 에러가 나요


import numpy as np
import pandas as pd
from pandas_datareader import data as web
import matplotlib.pyplot as plt
import matplotlib as mpl

tickers = ['AAPL', 'F', 'AMZN', 'GE', 'TSLA']

pxclose = pd.DataFrame()

for t in tickers:
    pxclose[t] = web.DataReader(t, data_source='yahoo', start='01-01-2019', end='31-12-2019')['Adj Close']

이렇게 하면


TypeError: string indices must be integers 에러가 납니다 도와주세요...