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