unable to cerate uniqie index on macaddr !

Hi,

create dhcp
(
mac macaddr,
ip  inet4
);

I fill it with unique mac,ip pairs and do

select * from dhcp;
...
(493 rows)

select * from dhcp group by mac;
...
(493 rows)

select distinct on mac mac from dhcp group by mac;
...
(493 rows)

create unique index dhcp_mac on dhcp (mac)
ERROR:  Cannot create unique index. Table contains non-unique values

Any clue ?
Table is freshly created;
Db is after a vacuum.

TIA,
Pawel Pierscionek