Using FireBird 1.0.0 796 on Windows 98

I have meta data like this

CREATE DOMAIN NUMERIC_17_2 AS
NUMERIC(17,2)
/* can be fill with null values */

CREATE TABLE TRANSAKSI (
ID NUMERIC(18,0) NOT NULL,
D_IMIP NUMERIC_17_2,
D_SVIP NUMERIC_17_2,
D_CDIP NUMERIC_17_2,
JUMLAH_DEBET COMPUTED BY (D_IMIP+D_SVIP+D_CDIP));

This table have two records with this value

D_IMIP D_SVIP D_CDIP
1000.26 null null
null 150.36 100.00

What i want is the calculated field jumlah_debet will give 1000.26 result,
but in this fields give me null results.

I try statement like this

select sum(d_imip)
give me 1000.26 results

select d_imip+d_svip+d_cdip from transaksi
give me null results

Any advice how to get results on this computed fields (remember that the
fields d_imip,d_svip,d_cdip can
fill with null values )

Thanks for any comments.

regards

Bayu