wsrep.foreign_key failed in buildbot on pre-testcase check

wsrep.foreign_key 'innodb_plugin'        w2 [ fail ]

        Test ended at 2017-12-13 16:34:39

CURRENT_TEST: wsrep.foreign_key

Could not execute 'check-testcase' before testcase 'wsrep.foreign_key' (res: 1):

mysqltest: Logging to '/dev/shm/var/2/tmp/check-mysqld_1.log'.

mysqltest: Results saved in '/dev/shm/var/2/tmp/check-mysqld_1.result'.

mysqltest: Connecting to server localhost:16060 (socket /dev/shm/var/tmp/2/mysqld.1.sock) as 'root', connection 'default', attempt 0 ...

mysqltest: ... Connected.

mysqltest: Start processing test commands from './include/check-testcase.test' ...

mysqltest: At line 78: query 'call mtr.check_testcase()' failed: 1047: WSREP has not yet prepared node for application use

not ok

mysqltest failed but provided no output