Crosstab Null Value Help

What's up?

I have a crosstab query with which i need help. The query takes all of the individual sales from 94 on and sorts them into total sales by year (the past 5 years 1999-2004) for each customer. After that, you can break the total sales for each customer down to the total sales for each category of product for the past 5 years. For example, you have ABC Co. that bought $20,000 last year total and bought $5,000 of category EN, 10,000 of category PS and 5,000 of category DS. My problem is that if ABC bought 5,000 in 2003 of EN but nothing of EN in 2004, the null value of 2004 does not appear as a zero, but as a blank. This obviously screws up my calculations and i need to know how to replace a null (blank) value with a zero. PLEASE HELP!!!!

if u need any more info i'll be happy to oblige.

thanks