Adding constraint to existing table.

I need some help with the following. I'm following script in 7.1.3 and 7.1.2

create table sales_sales (
salesid serial,
recid integer,
rep_id integer,
team_id integer,
process_date datetime,
total_lines integer,
total_revenue float4,
cancelled integer,
disconnected integer,
locked integer,
last_updated datetime,
updated_by varchar(15),
contract_date datetime,
clines integer,
cyear integer,
primary key ( salesid )
);

create table sales_sales_detail (
detid serial,
sales_id integer,
prod_id integer,
qty integer,
ext_price float4,
primary key ( detid )
);

create index recid29 on sales_sales ( recid );

create index sales_id30 on sales_sales_detail ( sales_id );

alter table sales_sales add constraint sales_sales_con foreign key
(salesid) references sales_sales_detail(sales_id) on delete cascade;

everything goes fine until I attempt to add the constraint. I get an error
that the unique key is not found. I know that a unique key is generated
with the serial definition.

What is wrong with my SQL statement.

Thanks.