Why is this UDF getting this error?

[JenniferMurphy]

The code below was working. Then I added the second parameter (pNumRevs). Now the statement tagged below gets an error.

Function AmazonRtg(pRtgs As String, _
          Optional pNumRevs As Double = 0, _
          Optional pCoef As Double = 0, _
          Optional pExp As Double = 0) As Variant

AmazonRtg = ""                        'Return null if pRtgRev not valid format

Const DefCoef As Double = 2.20296467  'The coefficient of the power function for alpha = 0.05
Const DefExp As Double = -0.5         'The exponent of the power function
Dim coef As Double    'The coefficient to be used
Dim exp As Double     'The exponent to be used
Dim Rtg As Double     'The rating to be used
Dim NumRevs As Double 'The number of ratings to be used
Dim arrParms As Variant               'The result of the split of rtg/#revs

If pCoef = 0 Then coef = DefCoef Else coef = pCoef   '<<< Error: Constant expression required
If pExp = 0 Then exp = DefExp Else exp = pExp

If InStr(pRtgs, "/") = 0 Then     'If rating and #reviews are separate,
  Rtg = pRtgs                       'Use the input parameters
  NumRevs = pNumRevs
Else                              'If they are combined, split them
  arrParms = Split(pRtgRevs, "/")   'Split the rating and the #reviews
  If UBound(arrParms) <> 1 Then Exit Function   'If not 2 values, return null
  If IsNumeric(arrParms(0)) And _
     IsNumeric(arrParms(1)) Then                'If both are numeric,
    Rtg = arrParms(0)
    NumRevs = arrParms(1)
  End If                                                    'Else, return null
End If

AmazonRtg = Rtg - coef * NumRevs ^ exp        'Return adjusted rating

End Function

It's called from this sheet:

Amazon Ratings.xlsx
	B	C	D
4	Rating	#Revs	Adj Rating
5	5.0	100	#VALUE!
Demo

Cell Formulas
Range	Formula
D5	D5	=AmazonRtg([@Rating],[@['#Revs]])

 

[JenniferMurphy]

I found the problem. When I added the new parameter, I changed the name of the first one from pRtgRevs to pRtgs. I filed to change that name in the Split statement. When I fixed that, it works. I do not understand why that should have caused the error of the earlier statement. (sigh)