[rules-users] "not" combined with predicate?

Does anyone know if there is a problem using a "not" combined with a predicate expression? Drools gives me a NullPointerException sometimes when evaluating my predicate. The example from the user guide is: Person( girlAge : age, sex = "F" ) Person( boyAge : age -> ( girlAge.intValue() == boyAge.intValue() + 2 ), sex = 'M' ) Buy I'm trying something similar to: Person( girlAge : age, sex = "F" ) not Person( boyAge : age -> ( girlAge.intValue() == boyAge.intValue() + 2 ), sex = 'M' ) And also: Person( girlAge : age, sex = "F" ) not Person( boyAge : age, sex = 'M' ) eval ( girlAge.intValue() == boyAge.intValue() + 2 ) Is there anything wrong with these? The compiler does not have a problem with the former, but does not like the latter. I'm using Drools 3.0.5. I'm basically trying to say "there is a girl, and there is not a boy whose age is 2 years less than the girl". Any thoughts on this would be greatly appreciated. Thanks, -Chris West