problem with uniques and foreing keys

Hi list...

Please i have a problem with this...

I create the follow tables...

--------------------------------------------------------

CREATE TABLE empresa (
id_empresa integer NOT NULL primary key,
nombre varchar(45),
);
CREATE TABLE casino (
id_casino integer NOT NULL,
id_empresa integer REFERENCES empresa(id_empresa),

nombre varchar(45),

primary key(id_casino,id_empresa)
);
CREATE TABLE maq_casino (
id_empresa integer NOT NULL REFERENCES casino(id_empresa),
id_casino integer NOT NULL REFERENCES casino(id_casino),

ubicacion_sala varchar(45) default NULL,
primary key(id_empresa,id_casino,id_tipo_maquina,id_maq_casino)
);

--------------------------------------------------------

When i'm gonna to create the last table i got this error:

ERROR: no hay restriccion unique que coincida con las columnas dadas en la
tabla referida <<casino>>

That in english is like .. there is no a unique constraint with columns
referred in casino table.

Please where is the problem...

Greetings ...

Krackem