I'm a fairly new user of VBA an I'm trying to write a simple function to do CAPM (finance). This is the first time I'm using arrays in VBA and I'm having a few problems although I've gotten the function to work. This is the working code:

Function myLogCAPM(index As Variant, stock As Variant, rf As Double, Rm As Double) As Double

    Dim var As Double
    Dim covar As Double
    Dim beta As Double
    Dim indexLog() As Double
    Dim stockLog() As Double
    Dim n As Byte
    Dim i As Byte
    
    n = index.Rows.Count
    n = n - 1
    
    ReDim indexLog(n) As Double
    ReDim stockLog(n) As Double

    For i = 1 To n
        indexLog(i) = Log(index(i, 1) / index(i + 1, 1))
        stockLog(i) = Log(stock(i, 1) / stock(i + 1, 1))
    Next i

    var = WorksheetFunction.var(indexLog)
    covar = WorksheetFunction.covar(indexLog, stockLog)
    beta = covar / var
    myLogCAPM = rf + beta * (Rm - rf)
    
End Function
But I don't like using datatype "Variant" and since all my values are numbers with a few decimals I can use "Double" with stock and index. But when I change this and get the following code it won't work!

Function myLogCAPM(index As Double, stock As Double, rf As Double, Rm As Double) As Double

    Dim var As Double
    Dim covar As Double
    Dim beta As Double
    Dim indexLog() As Double
    Dim stockLog() As Double
    Dim n As Byte
    Dim i As Byte
    
    n = index.Rows.Count
    n = n - 1
    
    ReDim indexLog(n) As Double
    ReDim stockLog(n) As Double

    For i = 1 To n
        indexLog(i) = Log(index(i, 1) / index(i + 1, 1))
        stockLog(i) = Log(stock(i, 1) / stock(i + 1, 1))
    Next i

    var = WorksheetFunction.var(indexLog)
    covar = WorksheetFunction.covar(indexLog, stockLog)
    beta = covar / var
    myLogCAPM = rf + beta * (Rm - rf)
    
End Function
I think the problem might be that theres not only numbers in my array. The input I give as index and stock is ONLY numbers but it seems for some reason that the cell above my inputs also comes into "index" and "stock" and that's the word "Open". To clarify some I give B2:B250 as index and it seems to also use B1. If I say "For i=0 to n" then the first value is "Open" and I thought that 0 was the first value of the array and 1 the second?

But I thought that I would check if this really is the problem so I insert instead a debug print to see but then it won't run and I get the feeling I'm fairly stuck.

Function myLogCAPM(index As Variant, stock As Variant, rf As Double, Rm As Double) As Double

    Dim var As Double
    Dim covar As Double
    Dim beta As Double
    Dim indexLog() As Double
    Dim stockLog() As Double
    Dim n As Byte
    Dim i As Byte
    
    n = index.Rows.Count
    n = n - 1
    
    ReDim indexLog(n) As Double
    ReDim stockLog(n) As Double

    For i = 1 To n
        indexLog(i) = Log(index(i, 1) / index(i + 1, 1))
        stockLog(i) = Log(stock(i, 1) / stock(i + 1, 1))
    Next i

    var = WorksheetFunction.var(indexLog)
    covar = WorksheetFunction.covar(indexLog, stockLog)
    beta = covar / var
    myLogCAPM = rf + beta * (Rm - rf)
    
    Debug.Print index 'Doesn't work with Debug.Print Ubound(index) either

End Function

Hopefully I made some sense in explaining the problem, I would appreciate some help!

Here's the excel file I'm doing it in.
CAPM.xlsm