[2017/03/12 20:51:22.994] python buildscripts/burn_in_tests.py --branch=master --buildVariant=enterprise-rhel-67-s390x --testListOutfile=jstests/new_tests.json --noExec $burn_in_args
 [2017/03/12 20:52:07.552] Comparing current branch against None
 [2017/03/12 20:52:07.553] Traceback (most recent call last):
 [2017/03/12 20:52:07.553]   File "buildscripts/burn_in_tests.py", line 468, in <module>
 [2017/03/12 20:52:07.553]     main()
 [2017/03/12 20:52:07.553]   File "buildscripts/burn_in_tests.py", line 431, in main
 [2017/03/12 20:52:07.553]     values.check_evergreen)
 [2017/03/12 20:52:07.553]   File "buildscripts/burn_in_tests.py", line 206, in find_changed_tests
 [2017/03/12 20:52:07.553]     revisions = callo(["git", "rev-list", base_commit + "..." + last_activated]).splitlines()
 [2017/03/12 20:52:07.553] TypeError: cannot concatenate 'str' and 'NoneType' objects
 [2017/03/12 20:52:07.588] Command failed: exit status 1
 [2017/03/12 20:52:07.588] Task completed - FAILURE.