What is the purpose of RightTupleIndexHashTable?

We are getting the exception below but unable to come up with a self contained unit test to demonstrate the problem.

Caused by: java.lang.NullPointerException: null
at org.drools.core.util.index.RightTupleIndexHashTable.removeAdd(RightTupleIndexHashTable.java:344) ~[drools-core-5.5.0.Final.jar:5.5.0.Final]
at org.drools.reteoo.JoinNode.modifyRightTuple(JoinNode.java:224) ~[drools-core-5.5.0.Final.jar:5.5.0.Final]
at org.drools.reteoo.PropagationQueuingNode$ModifyToSinkAction.execute(PropagationQueuingNode.java:513) ~[drools-core-5.5.0.Final.jar:5.5.0.Final]
at org.drools.reteoo.PropagationQueuingNode.propagateActions(PropagationQueuingNode.java:280) ~[drools-core-5.5.0.Final.jar:5.5.0.Final]
at org.drools.reteoo.PropagationQueuingNode$PropagateAction.execute(PropagationQueuingNode.java:588) ~[drools-core-5.5.0.Final.jar:5.5.0.Final]
at org.drools.common.AbstractWorkingMemory.executeQueuedActions(AbstractWorkingMemory.java:993) [drools-core-5.5.0.Final.jar:5.5.0.Final]