Using FindNext to Quantify Change in 2 Single-Word String Lists

Hi,
I'm trying to write a simple piece of code that establishes a cell position change in single word string lists. I have two lists with an identical set of string elements, arranged differently from one another. I use the findnext method to search for the first element of one column in the other column, and then when it is found I offset the value of the difference of the rows (i.e. the cell position change) into a third column. The code that I have so far works well in step by step debugging, but apparent iterations give me very large values. At this later stage, the debugger gives me an error 91 message pointing to the line: Loop While Not a Is Nothing And a.Address <> firstaddress. I have tried to use various for and do loop exits but without sucess. Does anyone have ideas of what could be done? Thank you.

Sub rankchng()
Dim rCell As Range
Dim firstaddress As String

Range("D2:D400").Clear
For Each rCell In Range("b2:b400")
If rCell = Empty Then Exit For

'Worksheets(Rankings).Range("a2:a400").Value = firstaddress
With Range("a2:a400")
Set a = .Find(rCell, LookIn:=xlValues)
If Not a Is Nothing Then
firstaddress = a.Address

Do
'If a = "" Then Exit Do
a.Offset(0, 3).Value = rCell.Row - a.Row

Set a = .findnext(a)

Loop While Not a Is Nothing And a.Address <> firstaddress
End If
End With

Next

End Sub