Not able to sum the field in Crystal reports 9 formula editor

Friends,

The below formula is not working. Please point me where iam making mistake.

VB Code:

whileprintingrecords;
global numbervar tottrd;
numbervar cnttottrd;
if 
(
{Admin.OTC}="001"
or 
{Admin.COEN} in 
["Chargeable Labour + Chargable Spares",
"Chargeable Labour + Free Spares",
"Chargeable Spares + Free Labour"]
)
then
cnttottrd := {PAYMENTS.PAIDAMT}
else
cnttottrd := 0;
tottrd := tottrd + cnttottrd

if i change
cnttottrd := {PAYMENTS.PAIDAMT}
to
cnttottrd := 1
then its working.......

Please treat this as urgent.

Thanks & Regards
Sathyguy

Can the PaidAmt field be Null? If yes, set the cnttottrd variable to 0 when the field is null.

Thanks.....yes there is a null value in that field.