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java.lang.NullPointerException at io.druid.query.aggregation.LongSumAggregator.combineValues(LongSumAggregator.java:39)[druid-processing-0.8.1.jar:0.8.1] at io.druid.query.aggregation.LongSumAggregatorFactory.combine(LongSumAggregatorFactory.java:78)[druid-processing-0.8.1.jar:0.8.1] at io.druid.segment.IndexMerger$RowboatMergeFunction.apply(IndexMerger.java:1242)[druid-processing-0.8.1.jar:0.8.1] at io.druid.segment.IndexMerger$RowboatMergeFunction.apply(IndexMerger.java:1218)[druid-processing-0.8.1.jar:0.8.1] at io.druid.collections.CombiningIterator.next(CombiningIterator.java:78)[druid-common-0.8.1.jar:0.8.1] at io.druid.segment.IndexMerger.makeIndexFiles(IndexMerger.java:730)[druid-processing-0.8.1.jar:0.8.1] at io.druid.segment.IndexMerger.merge(IndexMerger.java:336)[druid-processing-0.8.1.jar:0.8.1] at io.druid.segment.IndexMerger.mergeQueryableIndex(IndexMerger.java:218)[druid-processing-0.8.1.jar:0.8.1] at io.druid.segment.IndexMerger.mergeQueryableIndex(IndexMerger.java:207)[druid-processing-0.8.1.jar:0.8.1] at io.druid.segment.realtime.plumber.RealtimePlumber$4.doRun(RealtimePlumber.java:444)[druid-server-0.8.1.jar:0.8.1] at io.druid.common.guava.ThreadRenamingRunnable.run(ThreadRenamingRunnable.java:40)[druid-common-0.8.1.jar:0.8.1] at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)[?:1.8.0_60] at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)[?:1.8.0_60] at java.lang.Thread.run(Thread.java:745)[?:1.8.0_60]