urgent : postgresql question

Hi, sorry to bother you, but do you know what the following problem could
be?  I am trying to
create some relationships with the following SQL code in Postgresql:

ALTER TABLE dialup_account

        ADD CONSTRAINT customer_dialup_account

        FOREIGN KEY (

                customer_id

        )

        REFERENCES customer (

                customer_id

        );


I get the following error message:

ERROR:  parser: parse error at or near "constraint"

Any ideas?

Nabeel