Error substring Invalid length parameter passed to the substring function.

  Private Const constPurchaseIDPrefix As String = "PUR"
    Private Const constPurchaseCounterFormatPattern As String = "000000"

    'Overload for common usage
    Private Function GetPurchaseMatid() As String
        Return GetPurchaseMatid(DateTime.Now)
    End Function

    Private Function GetPurchaseMatid(ByVal RefDateTime As DateTime) As String

        Dim datePartStr As String = RefDateTime.ToString("ddMMyy")
        'Dicut only counter and if no record founded it's alway convert to 0 
        Dim retStr As String = String.Format("SELECT ISNULL(MAX(SUBSTRING([Pur_ID],{0},LEN([Pur_ID])-{1} )),0) FROM [Tl_Purchase]" _
            , constPurchaseIDPrefix.Length, datePartStr.Length)
       
        Dim d_command As SqlCommand = New SqlCommand(retStr, Conn)
        Dim PurchaseCounter As Integer = 0
        
        DGSTOCK.Visible = False
        Btnbuy.Visible = False
        Try
            Conn.Open()
            retStr = d_command.ExecuteScalar().ToString()
            PurchaseCounter = Convert.ToInt32(retStr) + 1
            'Response.Write(constPurchaseIDPrefix)
            'Response.Write(constPurchaseCounterFormatPattern)
            'Response.Write(datePartStr)
            Response.Write(retStr)
            Response.Write(PurchaseCounter)
            Response.End()
            retStr = String.Format("{0}{1}{2}", constPurchaseIDPrefix _
                , PurchaseCounter.ToString(constPurchaseCounterFormatPattern) _
                , datePartStr)
            Response.Write(retStr)
            Response.End()
            'Catch 'ex As Exception
            '    retStr = "#ERROR"
            'End Try
            'เป็น()
        Catch ex As Exception
            Throw New Exception(ex.Message)
        End Try

        Return retStr


    End Function