NullPointerException in Drools 5.1.1 - retractLeftTuple()

We recently switched to Drools 5.1.1 in our testing environments, and we are seeing a NullPointerException on a number of rule base / test case combinations. These all worked fine in 5.0.1.

The rule in question is relatively simple, but the rule base is complex.
It's difficult to send a test case, because the exception appears to occur only in large rule bases and layered rule flows.

We have had to roll back to Drools 5.0.1 and are unable to adopt 5.1.1 until this is resolved.

The relevant stack trace info is below.

com.wellsfargo.ARGenT.Execution.ARGenTRunTimeException - org.drools.runtime.rule.ConsequenceException: rule: RS7673.1.3_RF6365_83_50011728
...

Caused by:
rule: RS7673.1.3_RF6365_83_50011728
at org.drools.runtime.rule.impl.DefaultConsequenceExceptionHandler.handleException(DefaultConsequenceExceptionHandler.java:39)
at org.drools.common.DefaultAgenda.fireActivation(DefaultAgenda.java:927)
at org.drools.common.DefaultAgenda.fireNextItem(DefaultAgenda.java:856)
at org.drools.common.DefaultAgenda.fireAllRules(DefaultAgenda.java:1071)
at org.drools.common.AbstractWorkingMemory.fireAllRules(AbstractWorkingMemory.java:785)
at org.drools.common.AbstractWorkingMemory.fireAllRules(AbstractWorkingMemory.java:751)
at org.drools.impl.StatefulKnowledgeSessionImpl.fireAllRules(StatefulKnowledgeSessionImpl.java:218)
...

Caused by:
null
at org.drools.reteoo.RightInputAdapterNode.retractLeftTuple(RightInputAdapterNode.java:154)
at org.drools.reteoo.SingleLeftTupleSinkAdapter.doPropagateRetractLeftTuple(SingleLeftTupleSinkAdapter.java:217)
at org.drools.reteoo.SingleLeftTupleSinkAdapter.propagateRetractRightTuple(SingleLeftTupleSinkAdapter.java:124)
at org.drools.reteoo.JoinNode.retractRightTuple(JoinNode.java:151)
at org.drools.reteoo.ModifyPreviousTuples.retractTuples(ModifyPreviousTuples.java:132)
at org.drools.reteoo.EntryPointNode.modifyObject(EntryPointNode.java:176)
at org.drools.common.AbstractWorkingMemory.update(AbstractWorkingMemory.java:1442)
at org.drools.common.AbstractWorkingMemory.update(AbstractWorkingMemory.java:1349)
at org.drools.base.DefaultKnowledgeHelper.update(DefaultKnowledgeHelper.java:183)
at org.drools.base.DefaultKnowledgeHelper.update(DefaultKnowledgeHelper.java:196)
at com.wellsfargo.GeneratedRules.Rule_RS7673_1_3_RF6365_83_50011728_0.defaultConsequence(Rule_RS7673_1_3_RF6365_83_50011728_0.java:9)
at com.wellsfargo.GeneratedRules.Rule_RS7673_1_3_RF6365_83_50011728_0DefaultConsequenceInvoker.evaluate(Rule_RS7673_1_3_RF6365_83_50011728_0DefaultConsequenceInvoker.java:29)
at org.drools.common.DefaultAgenda.fireActivation(DefaultAgenda.java:917)
at org.drools.common.DefaultAgenda.fireNextItem(DefaultAgenda.java:856)
at org.drools.common.DefaultAgenda.fireAllRules(DefaultAgenda.java:1071)
at org.drools.common.AbstractWorkingMemory.fireAllRules(AbstractWorkingMemory.java:785)
at org.drools.common.AbstractWorkingMemory.fireAllRules(AbstractWorkingMemory.java:751)
at org.drools.impl.StatefulKnowledgeSessionImpl.fireAllRules(StatefulKnowledgeSessionImpl.java:218)
...

The line identified in in the exception from retractLeftTuple is in the for loop, and it references factHandle without first checking for null:

// retrieve handle from memory
final InternalFactHandle factHandle = (InternalFactHandle) memory.remove( tuple );

>>>>>> for ( RightTuple rightTuple = factHandle.getFirstRightTuple(); rightTuple != null; rightTuple = (RightTuple) rightTuple.getHandleNext() )

{ rightTuple.getRightTupleSink().retractRightTuple( rightTuple, context, workingMemory ); }

factHandle.setFirstRightTuple( null );