NullPointer when changing order of the rules

Hi there,

we are having some serious issues with some rules, they are throwing a NullPointerException and we don't understand why. I have tried to narrow down the problem to the smallest rule possible, now this rule doesn't really make much sense put like this but the real rule is more complex with more constraints. At the end the result is the same: a NPE.

I have tried it with many Drools versions from 5.x to latest 6.3.0-SNAPSHOT.

I tested this in isolation with the minimum amount of code possible, and attached it as well if someone wants to try it quickly.

Note that if line 2 of the when:
$a : Integer()
is moved as first line, the rule runs ok.

Please find the reproducer here: https://groups.google.com/forum/#!topic/drools-usage/-oNqu3l4cqE