Problem with null value in IDT.

Hi,

I am calculating the Sales Quantity like (Debit Qty- Credit Qty)

SUM(CASE WHEN 0DEB_CRED ='D'THEN 0QUANT_B END)-SUM(CASE WHEN 0DEB_CRED ='C'THEN 0QUANT_B END)

in Information Design Tool

In this when Credit value is null its taking Debit value also as null.

Ex: Debit is 10 and Credit is null its taking as 10-null= null.

Could you please suggest me how to resolve this.