NaN And Infinity Question

I have three fields in my detail row. The third of them divide the first two by passing them to a function:


=code.TestDivide(Fields!ThisYearPrice.Value, Fields!PriorYearPrice.Value)

Public Function TestDivide(Numerator as double, Divisor as double)
dim retVal
if Divisor = 0 then
retVal = 1
end if
if Numerator = 0 then
retVal = Numerator/Divisor
elseif Numerator< = 0 then
retVal= 0
else
retval =Numerator/Divisor
end if
return retVal
End Function


When Both Numerator and Divisor are 0, I get NaN
When the Divisor is 0, I get infinity.

I am trying this approach because I read about it in so many posts on this and other forums, but still having problems. What am I doing wrong?
Thanks!