KeyError in the example GW150914_advanced

When running the example GW150914_advanced.py (https://git.ligo.org/lscsoft/bilby/blob/master/examples/gw_examples/data_examples/GW150914_advanced.py), I encounter the error as below. File "/usr/local/lib/python3.7/site-packages/bilby-0.5.8-py3.7.egg/bilby/gw/waveform_generator.py", line 218, in parameters new_parameters.pop(key) KeyError: 'mass_2'

I would like to run the parameter estimation in terms of component masses. The error does not appear, when running GW150914.py.

I am using bilby 0.5.8 and python 3.7.