wsrep.pool_of_threads failed in buildbot with WSREP has not yet prepared node

wsrep.pool_of_threads                    w4 [ fail ]

        Test ended at 2017-05-08 20:52:23

CURRENT_TEST: wsrep.pool_of_threads

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

mysqltest: Logging to '/run/shm/var/4/tmp/check-mysqld_1.log'.

mysqltest: Results saved in '/run/shm/var/4/tmp/check-mysqld_1.result'.

mysqltest: Connecting to server localhost:16040 (socket /run/shm/var/tmp/4/mysqld.1.sock) as 'root', connection 'default', attempt 0 ...

mysqltest: ... Connected.

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

mysqltest: At line 81: query 'call mtr.check_testcase()' failed: 1047: WSREP has not yet prepared node for application use

not ok

mysqltest failed but provided no output