org.hibernate.exception.ConstraintViolationException on deleting process instances in jBPM3

Issue

I have tried to use the solution (supplied as a quickstart) in the following link Is there a way to automatically clean the jBPM tables from ended process instances?.

I get the following stack trace.

2013-07-01 12:18:00,729 ERROR [org.hibernate.util.JDBCExceptionReporter] (ESBScheduler:jbpm_maintenance.esb_Worker-0) Cannot delete or update a parent row: a foreign key constraint fails (`xxx`.`JBPM_TOKEN`, CONSTRAINT `FK_TOKEN_PROCINST` FOREIGN KEY (`PROCESSINSTANCE_`) REFERENCES `JBPM_PROCESSINSTANCE` (`ID_`))
2013-07-01 12:18:00,729 ERROR [org.hibernate.event.def.AbstractFlushingEventListener] (ESBScheduler:jbpm_maintenance.esb_Worker-0) Could not synchronize database state with session
org.hibernate.exception.ConstraintViolationException: could not delete: [org.jbpm.graph.exe.ProcessInstance#1903]
        at org.hibernate.exception.SQLStateConverter.convert(SQLStateConverter.java:94)
        at org.hibernate.exception.JDBCExceptionHelper.convert(JDBCExceptionHelper.java:66)
        at org.hibernate.persister.entity.AbstractEntityPersister.delete(AbstractEntityPersister.java:2559)
        at org.hibernate.persister.entity.AbstractEntityPersister.delete(AbstractEntityPersister.java:2715)
        at org.hibernate.action.EntityDeleteAction.execute(EntityDeleteAction.java:96)
        at org.hibernate.engine.ActionQueue.execute(ActionQueue.java:278)
        at org.hibernate.engine.ActionQueue.executeActions(ActionQueue.java:262)
        at org.hibernate.engine.ActionQueue.executeActions(ActionQueue.java:171)
        at org.hibernate.event.def.AbstractFlushingEventListener.performExecutions(AbstractFlushingEventListener.java:321)
        at org.hibernate.event.def.DefaultAutoFlushEventListener.onAutoFlush(DefaultAutoFlushEventListener.java:64)
        at org.hibernate.impl.SessionImpl.autoFlushIfRequired(SessionImpl.java:999)
        at org.hibernate.impl.SessionImpl.list(SessionImpl.java:1185)
        at org.hibernate.impl.QueryImpl.list(QueryImpl.java:102)
        at org.jbpm.db.GraphSession.deleteTasks(GraphSession.java:468)
        at org.jbpm.db.GraphSession.deleteProcessInstance(GraphSession.java:425)
        at org.jbpm.db.GraphSession.deleteProcessInstance(GraphSession.java:400)
        at org.jboss.soa.esb.samples.quickstart.jbpm.maintenance.DeleteOldJbpmProcessInstancesAction.deleteOldProcessInstance(DeleteOldJbpmProcessInstancesAction.java:135)
        at org.jboss.soa.esb.samples.quickstart.jbpm.maintenance.DeleteOldJbpmProcessInstancesAction.onSchedule(DeleteOldJbpmProcessInstancesAction.java:91)
        at org.jboss.soa.esb.listeners.ScheduleListener.onSchedule(ScheduleListener.java:121)
...
Caused by: com.mysql.jdbc.exceptions.jdbc4.MySQLIntegrityConstraintViolationException: Cannot delete or update a parent row: a foreign key constraint fails (`xxx`.`JBPM_TOKEN`, CONSTRAINT `FK_TOKEN_PROCINST` FOREIGN KEY (`PROCESSINSTANCE_`) REFERENCES `JBPM_PROCESSINSTANCE` (`ID_`))
        at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
        at sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:39)
        at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:27)
        at java.lang.reflect.Constructor.newInstance(Constructor.java:513)
        at com.mysql.jdbc.Util.handleNewInstance(Util.java:411)
        at com.mysql.jdbc.Util.getInstance(Util.java:386)
        at com.mysql.jdbc.SQLError.createSQLException(SQLError.java:1040)
        at com.mysql.jdbc.MysqlIO.checkErrorPacket(MysqlIO.java:3597)
        at com.mysql.jdbc.MysqlIO.checkErrorPacket(MysqlIO.java:3529)
        at com.mysql.jdbc.MysqlIO.sendCommand(MysqlIO.java:1990)
        at com.mysql.jdbc.MysqlIO.sqlQueryDirect(MysqlIO.java:2151)
        at com.mysql.jdbc.ConnectionImpl.execSQL(ConnectionImpl.java:2625)
        at com.mysql.jdbc.PreparedStatement.executeInternal(PreparedStatement.java:2119)
        at com.mysql.jdbc.PreparedStatement.executeUpdate(PreparedStatement.java:2415)
        at com.mysql.jdbc.PreparedStatement.executeUpdate(PreparedStatement.java:2333)
        at com.mysql.jdbc.PreparedStatement.executeUpdate(PreparedStatement.java:2318)
        at org.jboss.resource.adapter.jdbc.WrappedPreparedStatement.executeUpdate(WrappedPreparedStatement.java:365)
...

2013-07-01 12:18:00,731 ERROR [org.jboss.soa.esb.samples.quickstart.jbpm.maintenance.DeleteOldJbpmProcessInstancesAction] (ESBScheduler:jbpm_maintenance.esb_Worker-0) failed to delete process instance '1902': could not delete ProcessInstance(1902)