Can't use group by column alias

select b.n_name as nation, count(*) as cnt
  from customer_100 a, nation_100 b 
 where a.c_nationkey = b.n_nationkey 
 group by b.n_name
 order by cnt desc
 limit 7

2013-10-02 21:41:21,804 INFO  planner.PhysicalPlannerImpl (PhysicalPlannerImpl.java:createBestAggregationPlan(606)) - The planner chooses [Hash Aggregation]
2013-10-02 21:41:21,804 INFO  planner.PhysicalPlannerImpl (PhysicalPlannerImpl.java:createInMemoryHashAggregation(558)) - The planner chooses [Hash Aggregation]
2013-10-02 21:41:21,808 ERROR worker.Task (Task.java:run(389)) - java.lang.ArrayIndexOutOfBoundsException: 1
        at org.apache.tajo.engine.planner.physical.AggregationExec.<init>(AggregationExec.java:86)
        at org.apache.tajo.engine.planner.physical.HashAggregateExec.<init>(HashAggregateExec.java:48)
        at org.apache.tajo.engine.planner.PhysicalPlannerImpl.createInMemoryHashAggregation(PhysicalPlannerImpl.java:559)
        at org.apache.tajo.engine.planner.PhysicalPlannerImpl.createBestAggregationPlan(PhysicalPlannerImpl.java:607)
        at org.apache.tajo.engine.planner.PhysicalPlannerImpl.createGroupByPlan(PhysicalPlannerImpl.java:553)
        at org.apache.tajo.engine.planner.PhysicalPlannerImpl.createPlanRecursive(PhysicalPlannerImpl.java:148)
        at org.apache.tajo.engine.planner.PhysicalPlannerImpl.createPlanRecursive(PhysicalPlannerImpl.java:152)
        at org.apache.tajo.engine.planner.PhysicalPlannerImpl.createPlanRecursive(PhysicalPlannerImpl.java:169)
        at org.apache.tajo.engine.planner.PhysicalPlannerImpl.createPlan(PhysicalPlannerImpl.java:74)
        at org.apache.tajo.worker.TajoQueryEngine.createPlan(TajoQueryEngine.java:47)
        at org.apache.tajo.worker.Task.run(Task.java:378)
        at org.apache.tajo.worker.TaskRunner$2.run(TaskRunner.java:364)
        at java.lang.Thread.run(Thread.java:662)

2013-10-02 21:41:21,808 INFO  worker.Task (Task.java:run(434)) - Task Counter - total:11, succeeded: 10, failed: 1
2013-10-02 21:41:21,808 INFO  worker.TaskRunner (TaskRunner.java:run(307)) - Request GetTask: eb_1380717656859_0001_000004,container_1380717656859_0001_01_000040
2013-10-02 21:41:21,809 ERROR querymaster.QueryUnitAttempt (QueryUnitAttempt.java:transition(312)) - FROM gruter104.gruter.com >> java.lang.ArrayIndexOutOfBoundsException: 1
        at org.apache.tajo.engine.planner.physical.AggregationExec.<init>(AggregationExec.java:86)
        at org.apache.tajo.engine.planner.physical.HashAggregateExec.<init>(HashAggregateExec.java:48)
        at org.apache.tajo.engine.planner.PhysicalPlannerImpl.createInMemoryHashAggregation(PhysicalPlannerImpl.java:559)
        at org.apache.tajo.engine.planner.PhysicalPlannerImpl.createBestAggregationPlan(PhysicalPlannerImpl.java:607)
        at org.apache.tajo.engine.planner.PhysicalPlannerImpl.createGroupByPlan(PhysicalPlannerImpl.java:553)
        at org.apache.tajo.engine.planner.PhysicalPlannerImpl.createPlanRecursive(PhysicalPlannerImpl.java:148)
        at org.apache.tajo.engine.planner.PhysicalPlannerImpl.createPlanRecursive(PhysicalPlannerImpl.java:152)
        at org.apache.tajo.engine.planner.PhysicalPlannerImpl.createPlanRecursive(PhysicalPlannerImpl.java:169)
        at org.apache.tajo.engine.planner.PhysicalPlannerImpl.createPlan(PhysicalPlannerImpl.java:74)
        at org.apache.tajo.worker.TajoQueryEngine.createPlan(TajoQueryEngine.java:47)
        at org.apache.tajo.worker.Task.run(Task.java:378)
        at org.apache.tajo.worker.TaskRunner$2.run(TaskRunner.java:364)
        at java.lang.Thread.run(Thread.java:662)

2013-10-02 21:41:21,809 INFO  event.TaskScheduleEvent (TaskSchedulerImpl.java:handle(232)) - TaskRequest: container_1380717656859_0001_01_000040,eb_1380717656859_0001_000004
2013-10-02 21:41:21,809 INFO  querymaster.QueryUnit (QueryUnit.java:transition(432)) - =============================================================
2013-10-02 21:41:21,810 INFO  querymaster.QueryUnit (QueryUnit.java:transition(433)) - >>> Task Failed: ta_1380717656859_0001_000004_000000_00 <<<
2013-10-02 21:41:21,810 INFO  querymaster.QueryUnit (QueryUnit.java:transition(434)) - =============================================================