horizontal alignment

I had a macro in excel 2003 which rotated horizontal alignment of the selection.

Sub text_align()  'assign macro to Ctrl+q
    With Selection
        If .HorizontalAlignment = xlLeft Then
            .HorizontalAlignment = xlCenter
        ElseIf .HorizontalAlignment = xlCenter Then
            .HorizontalAlignment = xlRight
        ElseIf .HorizontalAlignment = xlRight Then
            .HorizontalAlignment = xlLeft
        End If
    End With
End Sub

However, in 2007 something changed. Now it seems there is a NULL problem and I can not get into the loop. I changed to this:

Sub text_align()  'assign macro to Ctrl+q
    With Selection
        If .HorizontalAlignment = xlLeft Or IsNull(.HorizontalAlignment) Then
            .HorizontalAlignment = xlCenter
        ElseIf .HorizontalAlignment = xlCenter Then
            .HorizontalAlignment = xlRight
        ElseIf .HorizontalAlignment = xlRight Then
            .HorizontalAlignment = xlLeft
        End If
    End With
End Sub

and now it only rotates between center and right (i never get left align). I use this for entire column selections (with headers) and with multiple range selections. Does anyone have any ideas. Does excel 2007 now see a selection range differently?

Thanks in advance
Heather