Table inheritance problem

Hello,
I have 3 tables: persons, operators and persons_position.

This is a semplified examples of their structures:

CREATE TABLE persons
(id varchar NOT NULL,
CONSTRAINT "PK_Persons" PRIMARY KEY(id));

CREATE TABLE operators
(id varchar NOT NULL,
CONSTRAINT "PK_Operators" PRIMARY KEY(id))
INHERITS(persons);

CREATE TABLE persons_position
(id bigserial NOT NULL,
person varchar NOT NULL);

and then there is a FOREIGN KEY CONSTRAINT from persons_position.person TO persons.id.

If I insert a tuple in operators...it results also in persons, but when I insert a tuple in persons_position, it says me I have violated the foreing key constraints. So it appears that the tuple really ISN'T in the persons table and the foreing key check fails.

How could I solve it, keeping the inheritance there?
Thanks