ERROR: plan should not reference subplan's variable

Hi *

I'm trying to run the following SQL statement on a PostgreSQL 8.1, installed
on a Windows machine:

INSERT INTO PROJECT(PROJECT_ID,PROJECT_DESC)
(
SELECT MAX(PROJECT_ID),'MYPROJECT'
FROM PROJECT
WHERE NOT EXISTS
(
SELECT PROJECT_DESC FROM PROJECT WHERE PROJECT_DESC = 'MYPROJECT'
)
)

and I get the following error:

ERROR: plan should not reference subplan's variable

If, for example, I replace MAX with some other aggregation (e.g. AVG or
SUM), everything works ok.

The table DDL looks like:

CREATE TABLE project
(
project_id int4 NOT NULL,
project_desc varchar(255),
CONSTRAINT project_pkey PRIMARY KEY (project_id)
)
WITH OIDS;

Do you have any clue why does this happen?

Thanks,
Catalin