Query Help

Hi All,

I am getting an error I do not understand from the following setup

CREATE TABLE timesheet_booking
(
timesheet_booking_id bigserial NOT NULL,
operator_id integer,
booking_item_id integer,
"day" date NOT NULL,
minutes integer NOT NULL,
CONSTRAINT timesheet_booking_pkey PRIMARY KEY (timesheet_booking_id),
CONSTRAINT timesheet_booking_booking_item_id_fkey FOREIGN KEY
(booking_item_id)
REFERENCES booking_item (booking_item_id) MATCH SIMPLE
ON UPDATE NO ACTION ON DELETE CASCADE,
CONSTRAINT timesheet_booking_operator_id_fkey FOREIGN KEY (operator_id)
REFERENCES "operator" (operator_id) MATCH SIMPLE
ON UPDATE NO ACTION ON DELETE SET NULL
)
WITHOUT OIDS;

CREATE UNIQUE INDEX timesheet_booking_unique_idx
ON timesheet_booking
USING btree
(operator_id, booking_item_id, "day");

Now the timesheet booking contains the following data:

timesheet_booking_id, operator_id,booking_item_id,day,minutes
2;284;1;"2007-01-18";10
4;284;2;"2007-01-18";10

If I try the following query:
delete from operator where operator_id=283;
I get the following error message:

ERROR: could not open relation with OID 438427
SQL state: XX000
Context: SQL statement "UPDATE ONLY "public"."timesheet_booking" SET
"operator_id" = NULL WHERE "operator_id" = $1"

If I try this sql statement directly it works OK. What does this error
mean and how can I get rid of it?

Thanks
Howard.