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Fwd: Hadoop Map Reduce Problem - Reduce failing

I've having a problem running my map/reduce job on my cluster. The map
phase, completes successfully, but the reduce phase fails with the
following error message:

Error: java.lang.NullPointerException
at java.util.concurrent.ConcurrentHashMap.get(ConcurrentHashMap.java:768)
at org.apache.hadoop.mapred.ReduceTask$ReduceCopier$GetMapEventsThread.getMapCompletionEvents(ReduceTask.java:2835)
at org.apache.hadoop.mapred.ReduceTask$ReduceCopier$GetMapEventsThread.run(ReduceTask.java:2756)

I've done a lot of searching, and I've found some solutions as to not having

any underscores in my hosts names. I changed them all, but the problem still

hasn't resolved.

Does anyone have any idea what the problem can be?