RuntimeWarning: divide by zero encountered in log when using pvlib

I'm using PVLib to model a PV system. I'm pretty new to coding and Python, and this is my first time using PVLib, so not surprisingly I've hit some difficulties.

Specifically, I've got created the following code using the extensive readthedocs examples at http://pvlib-python.readthedocs.io/en/latest/index.html

import pandas as pd
import numpy as np
from numpy import isnan
import datetime
import pytz

# pvlib imports
import pvlib
from pvlib.forecast import GFS, NAM, NDFD, HRRR, RAP
from pvlib.pvsystem import PVSystem, retrieve_sam
from pvlib.modelchain import ModelChain

# set location (Royal Greenwich Observatory, London, UK)
latitude, longitude, tz = 51.4769, 0.0005, 'Europe/London'

# specify time range.
start = pd.Timestamp(datetime.date.today(), tz=tz)
end = start + pd.Timedelta(days=5)
periods = 8 # number of periods that the GFS model and/or the model chain allows us to forecast power output.

# specify what irradiance variables we want
irrad_vars = ['ghi', 'dni', 'dhi']

# Use Global Forecast System model. The GFS is the US model that provides forecasts for the entire globe.
fx_model = GFS() # note: gives output in 3-hourly intervals

# retrieve data in processed format (convert temps from Kelvin to Celsius, combine elements of wind speed, complete irradiance data)
# Returns pandas.DataFrame object
fx_data = fx_model.get_processed_data(latitude, longitude, start, end)

# load module and inverter specifications
sandia_modules = pvlib.pvsystem.retrieve_sam('SandiaMod')
cec_inverters = pvlib.pvsystem.retrieve_sam('cecinverter')

module = sandia_modules['SolarWorld_Sunmodule_250_Poly__2013_'] 
inverter = cec_inverters['ABB__PVI_3_0_OUTD_S_US_Z_M_A__240_V__240V__CEC_2014_'] 

# model a fixed system in the UK. 10 strings of 250W panels, with 40 panels per string. Gives a nominal 100kW array
system = PVSystem(module_parameters=module, inverter_parameters=inverter, modules_per_string=40, strings_per_inverter=10)

# use a ModelChain object to calculate modelling intermediates
mc = ModelChain(system, fx_model.location, orientation_strategy='south_at_latitude_tilt')

# extract relevant data for model chain
mc.run_model(fx_data.index, weather=fx_data)

# OTHER CODE AFTER THIS TO DO SOMETHING WITH THE DATA

Having used a lot of print() statements in the console to debug, I can see that at the final line

mc.run_model(fx_data.index....

I get the following error:

/opt/pyenv/versions/3.6.0/lib/python3.6/site-packages/pvlib/pvsystem.py:1317: 
RuntimeWarning: divide by zero encountered in log
  module['Voco'] + module['Cells_in_Series']*delta*np.log(Ee) +
/opt/pyenv/versions/3.6.0/lib/python3.6/site-packages/pvlib/pvsystem.py:1323: 
RuntimeWarning: divide by zero encountered in log
  module['C3']*module['Cells_in_Series']*((delta*np.log(Ee)) ** 2) +

As a result, when I then go on to look at the ac_power outputs, I get what looks like erroneous data (every hour with a forecast that is not NaN = 3000 W).

I'd really appreciate any help you can give as I don't know what's causing it. Maybe I'm specifying the system incorrectly?

Thanks, Matt