Thanks Michal.
I'm using the drools distribution mentioned in the build.properties and build.xml that comes with version jbpm5.1.0M1. The drools version related to guvnor and the eclipse plugin is drools 5.2.0.M2 as far as i can see in build.properties. I haven't really touched drools.
Are you saying that the scenario above in jbpm5.1M1 does not cause you the exception?