Hello I'm getting an exception when adding a Collection<KnowledgePackage> after read it from a file with readObject() java.lang.NullPointerException at java.lang.Class.isAssignableFrom(Native Method) at org.drools.base.ClassObjectType.isAssignableFrom(ClassObjectType.java:180) at org.drools.reteoo.builder.PatternBuilder.attachPattern(PatternBuilder.java:95) at org.drools.reteoo.builder.PatternBuilder.build(PatternBuilder.java:80) at org.drools.reteoo.builder.GroupElementBuilder$AndBuilder.build(GroupElementBuilder.java:112) at org.drools.reteoo.builder.GroupElementBuilder.build(GroupElementBuilder.java:70) at org.drools.reteoo.builder.ReteooRuleBuilder.addSubRule(ReteooRuleBuilder.java:161) at org.drools.reteoo.builder.ReteooRuleBuilder.addRule(ReteooRuleBuilder.java:134) at org.drools.reteoo.ReteooBuilder.addRule(ReteooBuilder.java:113) at org.drools.reteoo.ReteooRuleBase.addRule(ReteooRuleBase.java:445) at org.drools.common.AbstractRuleBase.addRule(AbstractRuleBase.java:952) at org.drools.common.AbstractRuleBase.addPackages(AbstractRuleBase.java:629) *at org.drools.reteoo.ReteooRuleBase.addPackages(ReteooRuleBase.java:472)* at org.drools.impl.KnowledgeBaseImpl.addKnowledgePackages(KnowledgeBaseImpl.java:149) at andanta.conversacion.DroolsEmpresa.readKnowledgeBaseFromMemoryFile(DroolsEmpresa.java:304) at andanta.comunicacion.ServidorMR.cargaInfoConocimiento(ServidorMR.java:483) at andanta.test.Tester.main(Tester.java:127) The code for serialize the object is ObjectOutputStream out = new ObjectOutputStream( new FileOutputStream( fileName ) ); out.writeObject( kbuilder.getKnowledgePackages()); out.close(); And the code for deserialize is ObjectInputStream in = new ObjectInputStream( new FileInputStream( fileName) ); Collection<KnowledgePackage> kpkgs = (Collection<KnowledgePackage>)in.readObject(); in.close(); kbase = KnowledgeBaseFactory.newKnowledgeBase(); kbase.addKnowledgePackages( kpkgs ); My environment is eclipse 3.7, java 7.0.21 y drools 5.5.0. I thought this error may be due to a rule, but if I run kbase.addKnowledgePackages after reading the rules from drl file, there will be no exception. Best Regards. -- View this message in context: http://drools.46999.n3.nabble.com/Error-deserializing-Knowledge-Package-i... Sent from the Drools: User forum mailing list archive at Nabble.com.