drop constraint primary key

project=# select version();
version
-------------------------------------------------------------
PostgreSQL 7.2.1 on i686-pc-linux-gnu, compiled by GCC 2.96
(1 row)

Running above, am trying to drop the primary key, but get the follwing
error.

project=# alter table class_teacher_rlt drop constraint
class_teacher_rlt_pkey restrict;
ERROR: ALTER TABLE / DROP CONSTRAINT: class_teacher_rlt_pkey does not exist

project=# \d class_teacher_rlt
Table "class_teacher_rlt"
Column | Type | Modifiers
----------------------+--------------------------+--------------------------
--
class_id | integer | not null
user_id | integer | not null
school_id | integer | not null
start_date | timestamp with time zone | not null default now()
end_date | timestamp with time zone |
class_action_type_id | integer | not null default 1
action_detail | character varying(100) |
teacher_role_id | integer |
ind_prim_teacher | boolean | not null default
'f'::bool
ind_lead_teacher | boolean | default 'f'
status_id | integer | default 1
created_date | timestamp with time zone | not null default 'now'
last_modified | timestamp with time zone |
Primary key: class_teacher_rlt_pkey

Any ideas?

Thanks.

Marie