Modify block does not work with variables declared in the consequence

rule "test"
when
    $l : ArrayList() from collect (MyClass (attribute == false));
then
    for(Object o : new ArrayList( $l )) {
        MyClass o2 = (MyClass) o;
        modify(o2) { setAttribute(true) } 
    }
end

   modify( (MyClass)o) { setAttribute(true) }