NullPointerException occurs when a query includes an alias of UDF at the group by clause

SELECT hour(time) as t, sum(size) 
FROM log 
GROUP BY t  
ORDER BY t asc

13/10/08 21:21:18 ERROR worker.Task: java.lang.NullPointerException
        at org.apache.tajo.catalog.Schema.getColumnId(Schema.java:154)
        at org.apache.tajo.engine.eval.FieldEval.eval(FieldEval.java:51)
        at org.apache.tajo.engine.eval.GeneralFunctionEval.terminate(GeneralFunctionEval.java:62)
        at org.apache.tajo.engine.planner.physical.HashAggregateExec.next(HashAggregateExec.java:94)
        at org.apache.tajo.engine.planner.physical.MemSortExec.next(MemSortExec.java:54)
        at org.apache.tajo.engine.planner.physical.IndexedStoreExec.next(IndexedStoreExec.java:92)
        at org.apache.tajo.worker.Task.run(Task.java:381)
        at org.apache.tajo.worker.TaskRunner$2.run(TaskRunner.java:364)
        at java.lang.Thread.run(Thread.java:679)