Isolation=external result in core dump on 0.20.0

When upgrading from 0.19.1 to 0.20.0, any slaves started with the standard deimos setup fail hard on startup. The following command spits out about 20.000 errors before core dumping:

/etc/mesos-slave# /usr/local/sbin/mesos-slave --master=zk://localhost:2181/mesos --port=5051 --log_dir=/var/log/mesos --ip=172.17.8.101 --work_dir=/var/lib/mesos --isolation=external --containerizer_path=/usr/local/bin/deimos

output:

....
W0827 15:20:18.366271 721 containerizer.cpp:159] The 'external' isolation flag is deprecated, please update your flags to '--containerizers=external'.
W0827 15:20:18.366580 721 containerizer.cpp:159] The 'external' isolation flag is deprecated, please update your flags to '--containerizers=external'.
W0827 15:20:18.366631 721 containerizer.cpp:159] The 'external' isolation flag is deprecated, please update your flags to '--containerizers=external'.
W0827 15:20:18.366683 721 containerizer.cpp:159] The 'external' isolation flag is deprecated, please update your flags to '--containerizers=external'.
W0827 15:20:18.366714 721 containerizer.cpp:159] The 'external' isolation flag is deprecated, please update your flags to '--containerizers=external'.
W0827 15:20:18.366752 721 containerizer.cpp:159] The 'external' isolation flag is deprecated, please update your flags to '--containerizers=external'.
Segmentation fault (core dumped)