NPE in EqualityEvaluatorsDefinition$StringEqualEvaluator.evaluateCachedLeft

While working with rules with partitioning turned on.

KnowledgeBaseConfiguration config = KnowledgeBaseFactory.newKnowledgeBaseConfiguration();
config.setOption( MultithreadEvaluationOption.YES );
conf.setOption( MaxThreadsOption.get(5) );

Once we start inserting objects into the KSession, somewhere in the middle it throws this error:
Partition task manager caught an unexpected exception: null
Drools is capturing the exception to avoid thread death. Please report stack trace to development team.
java.lang.NullPointerException
at org.drools.base.evaluators.EqualityEvaluatorsDefinition$StringEqualEvaluator.evaluateCachedLeft(EqualityEvaluatorsDefinition.java:1984)
at org.drools.rule.VariableRestriction.isAllowedCachedLeft(VariableRestriction.java:109)
at org.drools.rule.VariableConstraint.isAllowedCachedLeft(VariableConstraint.java:115)
at org.drools.common.SingleBetaConstraints.isAllowedCachedLeft(SingleBetaConstraints.java:142)
at org.drools.reteoo.FromNode.checkConstraintsAndPropagate(FromNode.java:274)
at org.drools.reteoo.FromNode.assertLeftTuple(FromNode.java:138)
at org.drools.reteoo.SingleLeftTupleSinkAdapter.doPropagateAssertLeftTuple(SingleLeftTupleSinkAdapter.java:189)
at org.drools.reteoo.SingleLeftTupleSinkAdapter.propagateAssertLeftTuple(SingleLeftTupleSinkAdapter.java:64)
at org.drools.reteoo.JoinNode.assertLeftTuple(JoinNode.java:85)
at org.drools.reteoo.SingleLeftTupleSinkAdapter.doPropagateAssertLeftTuple(SingleLeftTupleSinkAdapter.java:189)
at org.drools.reteoo.SingleLeftTupleSinkAdapter.propagateAssertLeftTuple(SingleLeftTupleSinkAdapter.java:64)
at org.drools.reteoo.FromNode.checkConstraintsAndPropagate(FromNode.java:279)
at org.drools.reteoo.FromNode.assertLeftTuple(FromNode.java:138)
at org.drools.reteoo.SingleLeftTupleSinkAdapter.doPropagateAssertLeftTuple(SingleLeftTupleSinkAdapter.java:189)
at org.drools.reteoo.SingleLeftTupleSinkAdapter.propagateAssertLeftTuple(SingleLeftTupleSinkAdapter.java:64)
at org.drools.reteoo.JoinNode.assertObject(JoinNode.java:128)
at org.drools.reteoo.PartitionTaskManager$FactAssertAction.execute(PartitionTaskManager.java:274)
at org.drools.reteoo.PartitionTaskManager$PartitionTask.run(PartitionTaskManager.java:118)
at org.drools.concurrent.ExternalExecutorService$ObservableRunnable.run(ExternalExecutorService.java:453)
at java.util.concurrent.ThreadPoolExecutor$Worker.runTask(ThreadPoolExecutor.java:886)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:908)
at java.lang.Thread.run(Thread.java:662)