Is this already known and fixed in 5.4.0? The problem occurs when you have, e.g., Fact( foo.bar == "baz" ) Stack dump: Exception in thread "main" java.lang.NoClassDefFoundError: monitor/Monitor at ASMAccessorImpl_114212541328697624590.getValue(Unknown Source) at org.mvel2.optimizers.dynamic.DynamicGetAccessor.getValue(DynamicGetAccessor.java:73) at org.mvel2.ast.ASTNode.getReducedValueAccelerated(ASTNode.java:106) at org.mvel2.compiler.ExecutableAccessor.getValue(ExecutableAccessor.java:42) at org.mvel2.MVEL.executeExpression(MVEL.java:954) at org.drools.base.extractors.MVELClassFieldReader.getValue(MVELClassFieldReader.java:100) at org.drools.base.extractors.BaseObjectClassFieldReader.isNullValue(BaseObjectClassFieldReader.java:179) at org.drools.rule.VariableRestriction$ObjectVariableContextEntry.updateFromFactHandle(VariableRestriction.java:338) at org.drools.common.SingleBetaConstraints.updateFromFactHandle(SingleBetaConstraints.java:118) at org.drools.reteoo.NotNode.modifyRightTuple(NotNode.java:339) at org.drools.reteoo.BetaNode.modifyObject(BetaNode.java:431) at org.drools.reteoo.CompositeObjectSinkAdapter.doPropagateModifyObject(CompositeObjectSinkAdapter.java:468) at org.drools.reteoo.CompositeObjectSinkAdapter.propagateModifyObject(CompositeObjectSinkAdapter.java:436) at org.drools.reteoo.ObjectTypeNode.modifyObject(ObjectTypeNode.java:288) at org.drools.reteoo.EntryPointNode.modifyObject(EntryPointNode.java:271) at org.drools.common.NamedEntryPoint.update(NamedEntryPoint.java:459) at org.drools.common.NamedEntryPoint.update(NamedEntryPoint.java:363) at org.drools.base.DefaultKnowledgeHelper.update(DefaultKnowledgeHelper.java:298) at monitor.Rule_stop_monitoring.defaultConsequence(Rule_stop_monitoring.java:8) at monitor.Rule_stop_monitoringDefaultConsequenceInvoker.evaluate(Unknown Source) at org.drools.common.DefaultAgenda.fireActivation(DefaultAgenda.java:1091) at org.drools.common.DefaultAgenda.fireNextItem(DefaultAgenda.java:1029) at org.drools.common.DefaultAgenda.fireAllRules(DefaultAgenda.java:1251) at org.drools.common.AbstractWorkingMemory.fireAllRules(AbstractWorkingMemory.java:708) at org.drools.common.AbstractWorkingMemory.fireAllRules(AbstractWorkingMemory.java:672) at org.drools.common.Scheduler$ActivationTimerJob.execute(Scheduler.java:90) at org.drools.time.impl.DefaultTimerJobInstance.call(DefaultTimerJobInstance.java:46) at org.drools.time.impl.DefaultTimerJobInstance.call(DefaultTimerJobInstance.java:13) at org.drools.time.impl.PseudoClockScheduler.runCallBacks(PseudoClockScheduler.java:203) at org.drools.time.impl.PseudoClockScheduler.advanceTime(PseudoClockScheduler.java:156)