Comment 6 for bug 1217653

This problem arises when the referenced table (`a` in the ticket description) does not contain a PRIMARY KEY.
The easy workaround is to define PRIMARY KEY:

CREATE TABLE `a` (
  `id` bigint(20) DEFAULT NULL,
  PRIMARY KEY `id_pk` (`id`)
) ENGINE=InnoDB;

Galera documentation generally requires tables to have PKs.
The fix for the common case will be shortly.