Runtime Error 6' not sure why

[wwalsh3]

I'm doing a root finder using Newton's Method and it will run fine in my spreadsheet but when I do a step through i get an overflow runtime error and I'm not sure why. I've tried changing the variables from double to long but it doesn't seem to help.

Option Explicit

Sub NewtonMethod()


' Newton Method Macro to find roots of f(x)= 5-x-ln(x)
Dim xi As Double
' First guess of x
Dim xii As Double
' value of function with x = first guess
Dim xiii As Double
'value of first derivative of function with x = first guess
Dim tol As Double
' tolerance of calculation to see if solution fits into parameters
Dim i As Integer
' counter to stop loop
Dim imax As Integer
' maximim i value


' Initialize Variables
    Range("C3").Value = ""
' found root value will print to the right of the word Root in spreadsheet
    Range("C4").Value = ""
' number of itterations taken to find root will print to the right of the word Itterations in spreadsheet
    tol = Range("A2").Value
    xi = Range("A1").Value
    i = 0
    imax = 100




' Main


Do While (i < imax)
    ' xii = Root_Function(xi)
    xii = (xi - (Root_Function(xi) / Root_Deriv(xi)))           ' Error Here!
If (Abs((xi - xii) / xi) < tol) Then
    Range("C3").Value = xii
    Range("C4").Value = i
    Exit Do
Else
    xi = xii
    i = i + 1


    If (i = imax) Then MsgBox "Did not Converge"
    End If
Loop


End Sub


' Defines function given in terms of x
Function Root_Function(x As Double) As Double


Root_Function = 5 - x - Log(x)
' Root_Function = 5 - xi - Log(xi)


End Function


' Defines first derivative of function given
Function Root_Deriv(x As Double) As Double


Root_Deriv = -1# / x - 1#


End Function

I'm pretty new to this, any help would be appreciated.