bug?: permission denied for schema on "on delete set null"

I get a

Query failed: ERROR: permission denied for schema user_test CONTEXT:
SQL statement "UPDATE ONLY "user_test"."shop_commerce_baskets" SET
"sid" = NULL WHERE $1::pg_catalog.text OPERATOR(pg_catalog.=)
"sid"::pg_catalog.text

This query is run when I do a
DELETE FROM user_test.sessions WHERE timestamp < 1236672815;
as a result of an

on delete set null

but when I directly do a

update user_test.shop_commerce_baskets set sid=null;

I get no error.

create table user_test.sessions(
  sid int primary key,
);

create table user_test.shop_commerce_baskets (
  sid int references sessions (sid) on delete set null,
  ...
);

I'm on PostgreSQL 8.3.6 (Debian Lenny).

thanks

--
Ivan Sergio Borgonovo
http://www.webthatworks.it