TupleConstructor with named parameters

I have this Script:

def person = new Person(lastName: "foo", firstName: "bar")

println person

@groovy.transform.TupleConstructor  
@groovy.transform.ToString(includeNames = true, includeFields=true)  
class Person
{  
   def lastName  
   def firstName  
}

why it gives me:

Person(lastName:[lastName:foo, firstName:bar], firstName:null)

Why firstName remains null and lastName has the map of the parameters?

I am using groovy 1.8.6