Help With Divide By 0 Function

I'm new at SSRS and writing functions. I don't have something right here, but can't see what it is.
My function is:

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

I have to test both the numerator and divisor because either or both may be 0.

In the cell I call it with this formula, thinking that the black part is the first argument and the red part is the second. But SSRS tells me "Argument not specified for parameter "Divisor".

=Code.TestDivide(RunningValue(Fields!Sales.Value,sum,nothing) - RunningValue(Fields!Cost.Value,sum,nothing)) ,
RunningValue(Fields!Sales.Value,sum,nothing)

Can someone tell me what I'm doing wrong? Thanks!