vb.net - Visual Basic - Object reference not set to an instance of an object

I'm receiving the following error in VB.Net.

"Object reference not set to an instance of an object"

It highlights "Next" at the end of the For Loop.

Any help would be great.

Imports System.IO
Public Class LoginForm
    Dim Username() As String

    Dim Password() As String
    Dim Index As Integer

    Public Function encrypt(ByVal data As String) As String
        Dim answer As String = ""
        Dim I As Integer
        data = RTrim(data)
        If Mid(data, 1, 1) <> Chr(0) Then
            For I = 1 To Len(data)
                answer = answer + Chr(Asc(Mid(data, I, 1)) Xor 23) 

               ' Xor 23 is a simple encription cipher, a string can be 
               ' encrypted or de-encrypted by the value following the Xor 
               'i.e. "23" '
            Next I
        End If
        encrypt = answer
    End Function

    Private Sub LoginButton_Click(ByVal sender As System.Object, _
                                    ByVal e As System.EventArgs) _

                                Handles LoginButton.Click

        For Each I In Username

            If UserNameTextBox.Text = Username(Index) Then
                UserAdd.Show()
                Me.Hide()
                If PasswordTextBox.Text = Password(Index) Then
                    MessageBox.Show("Correct Password")
                Else

                    MessageBox.Show("Invalid Password, Sorry")
                End If
            Else : MessageBox.Show("Invalid Username, Sorry")
            End If    
        Next    
    End Sub

    Public Sub ReadUsers()
        Dim CurrentFileReader As StreamReader
        Dim FileName, Line As String

        Dim Delimiter As Char = ","
        Dim Feild() As String
        Dim Username() As String
        Dim Password() As String
        Dim Index As Integer

        FileName = "C:\Computing\Projects\Login\Users.txt"  'location of 
                                                            'user file 
        CurrentFileReader = New StreamReader(FileName)


        Do Until CurrentFileReader.EndOfStream

            Line = CurrentFileReader.ReadLine
            If Line = Nothing Then
                Exit Do
            End If

            ReDim Preserve Username(Index)
            ReDim Preserve Password(Index)


            Feild = Line.Split(Delimiter)

            Username(Index) = encrypt(Feild(0))
            Password(Index) = encrypt(Feild(1))        
        Loop
    End Sub        

    Private Sub LoginForm_Load(ByVal sender As Object, _
                                ByVal e As System.EventArgs) _
                            Handles Me.Load

        Call ReadUsers()
    End Sub
End Class