VBA - runtime error 94, invalid use of null

Hi There

I've been using this macro that allows me to select multiple items in a drop down list

Option Explicit
Public Prior As String

Private Sub Worksheet_Change(ByVal Target As Range)
'Author:    Jerry Beaucaire
'Date:      10/2/2009, 8/17/2010
'Summary:   These two macros create a multi-choice dropbox
'           This version allows each option to be selected only once
If Target.Count > 1 Then Exit Sub
Application.EnableEvents = False
    If Not Intersect(Target, Range("E:E")) Is Nothing Then
        If Target <> "" Then
            If Prior <> "" Then
                If InStr(Prior, Target) > 0 Then
                    Application.Undo
                Else
                    Target = Target.Text & "," & Prior
                    Prior = Target
                End If
            End If
        Else
            Prior = ""
        End If
    End If
Application.EnableEvents = True
End Sub


Private Sub Worksheet_SelectionChange(ByVal Target As Range)
    Prior = Target.Text
End Sub

When I run debug it says that there is something to do with the Prior=Target.Text line. I'm a bit out of my depth here - how do I go about debugging this? I've attached my spreadsheet as an example. To get the error to occur, simply continue using the heading that states 'tax concession sought' selecting different items. It sometimes happens straight away, and other times it doesn't happen for a while, but eventually it does...

Register Null Error.xlsm

Thanks in advance

Arneld

dude you need to make a selection in c column for it to work

Of course, but regardless of that, the error still occurred. Cheers for your response anyway!

It's working perfectly now. Thank you so much for your help!