foreign key problem

Dear users,
I'm trying to create constraints into a db they give me from access...
I'm using pgadmin to create a new foreign key for the relationship
between two table..

I have a table called "generalita" wich contains a column called
"cod_carg":
___________________
select cod_carg from generalita group by cod_carg;
cod_carg
----------

2
0
1
(4 righe)
___________________

I have another table, called "diz_carg" that contain, into a column
called "cod_carg" (that is a primary key), the domain values for the
"cod_carg" column of "generalita":
__________________
select cod_carg from diz_carg group by cod_carg;
cod_carg
----------
1
0
2
(3 righe)
__________________

so when I try to create the foreign key I obtain:
ALTER TABLE generalita ADD CONSTRAINT fkey_diz_cod_carg FOREIGN KEY
(cod_carg) REFERENCES diz_carg (cod_carg) ON UPDATE NO ACTION ON DELETE
NO ACTION;

____________________________
ERROR: insert or update on table "generalita" violates foreign key
constraint "fkey_diz_cod_carg"
DETAIL: Key (cod_carg)=( ) is not present in table "diz_carg".
____________________________

can I allow null values for "generalita.cod_carg" although it is a
foreign key for "diz_carg.cod_carg"?

I apologize if it is a stupid question.... :-(

many thanks to all

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Ivan Marchesini
Department of Civil and Environmental Engineering
University of Perugia
Via G. Duranti 93/a
06125
Perugia (Italy)
e-mail: marchesini(at)unipg(dot)it
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