[rules-users] NPE in RightTuple

Does anyone know what this would be indicative of? I've seen it occur periodically at which point I have to discard the session. I flatten and assert my data, run the rules, and then retract the handles collected on assertion. I've ensured the fact handle parameter is never null. I have the stateful session wrapped in a synchronized business façade, so I don't think it should be a concurrency issue (unless I have a bug of course). I'm running Drools 5 M4. java.lang.NullPointerException at org.drools.util.RightTupleIndexHashTable.remove(RightTupleIndexHashTable.java:224) at org.drools.reteoo.JoinNode.retractRightTuple(JoinNode.java:204) at org.drools.reteoo.ObjectTypeNode.retractObject(ObjectTypeNode.java:210) at org.drools.reteoo.EntryPointNode.retractObject(EntryPointNode.java:190) at org.drools.common.AbstractWorkingMemory.retract(AbstractWorkingMemory.java:1078) at org.drools.common.AbstractWorkingMemory.retract(AbstractWorkingMemory.java:1045)