creating an index on numeric type

Hi all,

I couldn't create a table which has a primary key on numeric type.

create table t (id numeric(7,2) primary key);
NOTICE:  CREATE TABLE/PRIMARY KEY will create implicit index
't_pkey' for table 't'
ERROR:  Can't find a default operator class for type 1700. 

How can I create an index on numeric type ? 

Regards.

Hiroshi Inoue
Inoue@tpf.co.jp