Hi all, I'm migrating from an old Drools 5.1 version to 6.0.0.Final. Sometimes (I don't really get under which conditions as sometimes it works), I'm facing an InstantiationError exception when I insert a fact into a stateful kie session. This exception is thrown by a generated class named ConditionEvaluator<UUID> which doesn't help for debugging! ;) The inserted object is a basic POJO (named JasmineEventEB) except for an attribute named "value" which accept any "Serializable" object and which is causing the issue. The rule condition is quite simple $e : JasmineEventEB(probe == "Button:pris" && value == "1") In debugging I found that it failed when evaluating the value == "1" part. I can also confirm that the value for the actually inserted JasmineEventEB is a string. The stacktrace is: java.lang.InstantiationError: java.io.Serializable at ConditionEvaluatorae1a73837e8146bda23004a0450e2e52.evaluate(Unknown Source) at org.drools.core.rule.constraint.MvelConstraint.evaluate(MvelConstraint.java:212) at org.drools.core.rule.constraint.MvelConstraint.isAllowed(MvelConstraint.java:169) at org.drools.core.reteoo.AlphaNode.assertObject(AlphaNode.java:134) at org.drools.core.reteoo.CompositeObjectSinkAdapter.doPropagateAssertObject(CompositeObjectSinkAdapter.java:502) at org.drools.core.reteoo.CompositeObjectSinkAdapter.propagateAssertObject(CompositeObjectSinkAdapter.java:387) at org.drools.core.reteoo.AlphaNode.assertObject(AlphaNode.java:138) at org.drools.core.reteoo.CompositeObjectSinkAdapter.doPropagateAssertObject(CompositeObjectSinkAdapter.java:502) at org.drools.core.reteoo.CompositeObjectSinkAdapter.propagateAssertObject(CompositeObjectSinkAdapter.java:377) at org.drools.core.reteoo.ObjectTypeNode.assertObject(ObjectTypeNode.java:288) at org.drools.core.reteoo.EntryPointNode.assertObject(EntryPointNode.java:260) at org.drools.core.common.NamedEntryPoint.insert(NamedEntryPoint.java:360) at org.drools.core.common.NamedEntryPoint.insert(NamedEntryPoint.java:279) at org.drools.core.common.AbstractWorkingMemory.insert(AbstractWorkingMemory.java:1149) at org.drools.core.common.AbstractWorkingMemory.insert(AbstractWorkingMemory.java:1093) at org.drools.core.impl.StatefulKnowledgeSessionImpl.insert(StatefulKnowledgeSessionImpl.java:308) I tested both with the java and mvel dialect with the same result. Does someone has already encountered this kind of error? If you need more details or clarifications please let me know. Thanks & regards, Loïc