Pig OrcStorage fail to load a map with null key

Error message:

Backend error message
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AttemptID:attempt_1403634189382_0006_m_000000_1 Info:Error: java.lang.NullPointerException
at org.apache.pig.impl.util.orc.OrcUtils.convertOrcToPig(OrcUtils.java:97)
at org.apache.pig.impl.util.orc.OrcUtils.convertOrcToPig(OrcUtils.java:82)
at org.apache.pig.builtin.OrcStorage.getNext(OrcStorage.java:312)
at org.apache.pig.backend.hadoop.executionengine.mapReduceLayer.PigRecordReader.nextKeyValue(PigRecordReader.java:211)
at org.apache.hadoop.mapred.MapTask$NewTrackingRecordReader.nextKeyValue(MapTask.java:533)
at org.apache.hadoop.mapreduce.task.MapContextImpl.nextKeyValue(MapContextImpl.java:80)
at org.apache.hadoop.mapreduce.lib.map.WrappedMapper$Context.nextKeyValue(WrappedMapper.java:91)
at org.apache.hadoop.mapreduce.Mapper.run(Mapper.java:144)
at org.apache.hadoop.mapred.MapTask.runNewMapper(MapTask.java:764)
at org.apache.hadoop.mapred.MapTask.run(MapTask.java:340)
at org.apache.hadoop.mapred.YarnChild$2.run(YarnChild.java:167)
at java.security.AccessController.doPrivileged(Native Method)
at javax.security.auth.Subject.doAs(Subject.java:415)
at org.apache.hadoop.security.UserGroupInformation.doAs(UserGroupInformation.java:1557)
at org.apache.hadoop.mapred.YarnChild.main(YarnChild.java:162)