"plan should not reference subplan's variable" when using row level security

Hi,

Whilst trying to use row level security with a subquery in the USING
expression, I'm receiving an error "plan should not reference
subplan's variable"

A simple sql file to reproduce:

****

CREATE TABLE a (
    id      INTEGER PRIMARY KEY
);

CREATE TABLE b (
    id      INTEGER PRIMARY KEY,
    a_id    INTEGER,
    text    TEXT
);

CREATE POLICY a_select ON b FOR SELECT
    USING ( EXISTS(SELECT FROM a WHERE a.id = b.a_id) );

ALTER TABLE b ENABLE ROW LEVEL SECURITY;

INSERT INTO a (id) VALUES (1);

INSERT INTO b (id, a_id, text) VALUES (1, 1, 'one');

GRANT ALL ON ALL TABLES IN SCHEMA public TO test;

SET ROLE test;

SELECT * FROM b;

UPDATE b SET text = 'ONE' WHERE id = 1;

****

gives error:

psql:/tmp/test.sql:26: ERROR:  plan should not reference subplan's variable

Is this a bug or am I doing something wrong?

Any help much appreciated,

Adam