[rules-users] Invalid package exception

I have a utility that compiles a package then serializes it to disk in a completely standard fashion. Then, in the application, the package is deserialized to construct a rule base. Now, this works well except if I add a rule flow to it (from a .rfm file) - then I get the following exception on deserialization (original compilation of the package and serialization goes okay): org.drools.rule.InvalidRulePackage at org.drools.rule.Package.checkValidity(Package.java:424) at org.drools.common.AbstractRuleBase.addPackage(AbstractRuleBase.java:364) Anyone know what might be going on? Regards, M.