Using TimeBombMakeReadOnly gives run-time error?

I am using Chip Pearson's code for setting a timebomb. However I am getting a run-time error '1004' Application-defined or object-defined error when the ExpirationDate is set on the second line of execution. Shouldn't this run as is or am I missing something here? I don't get why the error isn't handled with the "On Error" code ...

Sub TimeBombMakeReadOnly()
''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''
' TimeBombMakeReadOnly
' This procedure uses a defined name to store the expiration
' date and if the workbook has expired, makes the workbook
' read-only.
'
' Source: http://www.cpearson.com/excel/workbooktimebomb.aspx
''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''

    Dim ExpirationDate As String
    Dim NameExists As Boolean

    On Error Resume Next
    ExpirationDate = Mid(ThisWorkbook.Names("ExpirationDate").Value, 2)
    If Err.Number <> 0 Then
        '''''''''''''''''''''''''''''''''''''''''''
        ' Name doesn't exist. Create it.
        '''''''''''''''''''''''''''''''''''''''''''
        ExpirationDate = CStr(DateSerial(Year(Now), _
                                         Month(Now), Day(Now) + C_NUM_DAYS_UNTIL_EXPIRATION))
        ExpirationDate = CStr(DateSerial(Year(C_WORKBOOK_ISSUE_DATE), _
                                         Month(C_WORKBOOK_ISSUE_DATE), Day(C_WORKBOOK_ISSUE_DATE) + C_NUM_DAYS_UNTIL_EXPIRATION))
        ThisWorkbook.Names.Add Name:="ExpirationDate", _
                               RefersTo:=Format(ExpirationDate, "short date"), _
                               Visible:=False
        NameExists = False
    Else
        NameExists = True
    End If

    ''''''''''''''''''''''''''''''''''''''''''''''''''''''
    ' If the today is past the expiration date, make the
    ' workbook read only. We need to Save the workbook
    ' to keep the newly created name intact.
    ''''''''''''''''''''''''''''''''''''''''''''''''''''''
    If CDate(Now) >= CDate(ExpirationDate) Or CDate(Now) < CDate(C_WORKBOOK_ISSUE_DATE) Then
        If NameExists = False Then
            ThisWorkbook.Save
        End If
        ThisWorkbook.ChangeFileAccess xlReadOnly
    End If
    ThisWorkbook.Names.Add.Delete
End Sub

It seems that this code was written to accommodate the pre-existing definition of the name (hence the On Error Resume Next handling). Testing this routine in a brand new workbook causes the same error, so I don't see how this can be the problem.