NPE in MapReduceTask reduce phase

During the execution of a MapReduce word count job with 6 nodes, the following NPE is thrown:

11:19:37,870 ERROR [org.infinispan.remoting.InboundInvocationHandlerImpl] (remote-thread-2) Exception executing command
java.lang.NullPointerException
at org.infinispan.distexec.mapreduce.MapReduceManagerImpl.reduce(MapReduceManagerImpl.java:153)
at org.infinispan.commands.read.ReduceCommand.perform(ReduceCommand.java:88)
at org.infinispan.remoting.InboundInvocationHandlerImpl.handleInternal(InboundInvocationHandlerImpl.java:122)
at org.infinispan.remoting.InboundInvocationHandlerImpl.access$000(InboundInvocationHandlerImpl.java:68)
at org.infinispan.remoting.InboundInvocationHandlerImpl$2.run(InboundInvocationHandlerImpl.java:194)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1145)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:615)
at java.lang.Thread.run(Thread.java:724)

The full log is here - https://jenkins.mw.lab.eng.bos.redhat.com/hudson/user/afield@REDHAT.COM/my-views/view/afield's%20jobs/job/jdg-radargun-mapreduce-test/181/console-edg-perf06/

Looking at the code to see if I can figure out what happened.