ValueError: Parameter `start` received with timezone defined as 'UTC' although a Date must be timezone naive

OLA. Trying to run the code from chapter 8 of zipline but the following error is occurring.

start = time()
results = run_algorithm(start=start_date,
end=end_date,
initialize=initialize,
before_trading_start=before_trading_start,
capital_base=1e6,
data_frequency=‘daily’,
bundle=‘quandl’,
custom_loader=signal_loader) # need to modify zipline

print(‘Duration: {:.2f}s’.format(time() - start))

start_date AND end_date ARE IN THE FOLLOWING FORMAT.
(Timestamp(‘2014-12-09 00:00:00+0000’, tz=‘UTC’),
Timestamp(‘2017-11-30 00:00:00+0000’, tz=‘UTC’))

Could anyone help me? Thanks