How do I fix this error "a boolean array is required here" in Crystal report?

I use Crystal Report 8.5
I just want to add a formular field to the report but the database fields in my formular contain null value. I've tried the convert null to default value in the Report Options. But it doesn't work. It makes my report shows no result at all. So someone suggested me to convert the null field by creating a newformular field of that null field using if then else.
i.e. database field that contain null are A, B, C, D.
I want to make a new report field that get the value of A+B+C-D
what I did was:
@A = if isnull(A) then 0.00 else A = A
and the same for @B, @C, and @D
@WDV = @A + @B + @C - @D  
But then I come across this error "a boolean array is required here" and it show the ( at the @B
How do I get this right?