Showing Zero if field is null

Hi All,

I have a report having qty fields.I need to show as zero when value is null.

So wrote a formula like this.

@Target Qty:

if is null(qty) and is null(act qty) then 0 else qty * act qty.

I placed this is in detail section and working fine.

Then I created a summary sum(@Target Qty) for this.This also should show as zero when the qty fields are null.

I placed this in report footer .Whenever i kept this in report the @Target Qty which is in detail section is hiding.If i removed summary formula then @Target Qty is showing.

Please suggest what is the problem