Null values convert to 0 - Crystal Report

When there are some null values in the "Running Total Fields" column, i should print 0(Zero);
I try first method:

IIF(not isnull({#RPay}),{#RPay},0)

but it cannot work.

Finally, I change it like the following code:

currencyVar cTotal := 0;
if not isnull({#RPay}) then cTotal := cTotal + {#RPay};
cTotal