I am pretty sure this is a bug, but would like confirmation from people more familiar with the code base. My tests seem to show that RuleTerminalNode can cause an NPE on two different lines of code. Both places where a call is made to "workingMemory.getTruthMaintenanceSystem()" in that class can result in a null pointer exception if the "maintainTms" option is false. In my case I discovered the issue in some unit tests where I am calling "knowledgeBase.removeKnowledgePackage" which ends up (eventually) in RuleTerminalNode. I have pasted an example stack trace below: Thread [main] (Suspended (exception NullPointerException)) RuleTerminalNode$RTNCleanupAdapter.cleanUp(LeftTuple, InternalWorkingMemory) line: 519 ObjectTypeNode.doRemove(RuleRemovalContext, ReteooBuilder, BaseNode, InternalWorkingMemory[]) line: 347 ObjectTypeNode.remove(RuleRemovalContext, ReteooBuilder, BaseNode, InternalWorkingMemory[]) line: 325 AlphaNode(ObjectSource).doRemove(RuleRemovalContext, ReteooBuilder, BaseNode, InternalWorkingMemory[]) line: 209 AlphaNode(BaseNode).remove(RuleRemovalContext, ReteooBuilder, BaseNode, InternalWorkingMemory[]) line: 109 LeftInputAdapterNode.doRemove(RuleRemovalContext, ReteooBuilder, BaseNode, InternalWorkingMemory[]) line: 187 LeftInputAdapterNode(BaseNode).remove(RuleRemovalContext, ReteooBuilder, BaseNode, InternalWorkingMemory[]) line: 109 RuleTerminalNode.doRemove(RuleRemovalContext, ReteooBuilder, BaseNode, InternalWorkingMemory[]) line: 411 RuleTerminalNode(BaseNode).remove(RuleRemovalContext, ReteooBuilder, BaseNode, InternalWorkingMemory[]) line: 109 ReteooBuilder.removeRule(Rule) line: 237 ReteooRuleBase.removeRule(Rule) line: 432 ReteooRuleBase(AbstractRuleBase).removeRule(Package, Rule) line: 831 ReteooRuleBase(AbstractRuleBase).removePackage(String) line: 751 KnowledgeBaseImpl.removeKnowledgePackage(String) line: 201 (I redacted the stack after the call to removeKnowledgePackage() because it provides no value in this discussion) I am using Drools 5.1.1. I have also looked at the latest version of the code here: https://github.com/droolsjbpm/droolsjbpm/blob/master/drools-core/src/mai n/java/org/drools/reteoo/RuleTerminalNode.java Can anyone confirm that this is a bug, or if not tell me what I am doing wrong? Thank You, Nathan Bell