[rules-users] org.drools.reteoo.AlphaNode$AlphaMemory cannot be cast to org.drools.reteoo.BetaMemory

Can anyone explain what this ConsequenceException means? Exception in thread "main" org.drools.runtime.rule.ConsequenceException: java.lang.ClassCastException: org.drools.reteoo.AlphaNode$AlphaMemory cannot be cast to org.drools.reteoo.BetaMemory at org.drools.runtime.rule.impl.DefaultConsequenceExceptionHandler.handleEx ception(DefaultConsequenceExceptionHandler.java:23) at org.drools.common.DefaultAgenda.fireActivation(DefaultAgenda.java:943) at org.drools.common.DefaultAgenda.fireNextItem(DefaultAgenda.java:885) at org.drools.common.DefaultAgenda.fireAllRules(DefaultAgenda.java:1086) at org.drools.common.AbstractWorkingMemory.fireAllRules(AbstractWorkingMemo ry.java:660) at org.drools.common.AbstractWorkingMemory.fireAllRules(AbstractWorkingMemo ry.java:627) at org.drools.impl.StatefulKnowledgeSessionImpl.fireAllRules(StatefulKnowle dgeSessionImpl.java:183) at org.jcvi.annotation.rulesengine.RulesEngine.fireAllRules(RulesEngine.jav a:210) at org.jcvi.annotation.Aruba.run(Aruba.java:311) at org.jcvi.annotation.RunTest.main(RunTest.java:55) Nelson