AutoNumber ไม่ทำงาน ขึ้น Error Input string was not in a correct format.

Public Function order_id() As String
        Dim SqlCon As New SqlConnection(con)
        Dim sb As New StringBuilder
        Dim intNum As String = ""
        Dim id As String = ""
        Dim ids As String = ""
        sb.Append("SELECT MAX(orderID) AS orderID FROM OrderTest")
        intNum = sb.ToString
        Dim da As New SqlDataAdapter(intNum, SqlCon)
        Dim ds As New DataSet
        da.Fill(ds, "OrderTest")
        SqlCon.Open()
        If IsDBNull(ds.Tables("OrderTest").Rows(0)("orderID")) Then
            id = "OR" & CStr(Now.Year + 543).Substring(2) & "00001"
        Else
            ids = "OR" & (CInt(ds.Tables("OrderTest").Rows(0)("orderID").Substring(4)))
           ids = ids + 1 ' Error ตรงนี้ Input string was not in a correct format.

            Dim CountID As Integer = Len(ids)
            If CountID = 5 Then
                id = "OR" & CStr(Now.Year + 543).Substring(2) & ids
            ElseIf CountID = 4 Then
                id = "OR" & CStr(Now.Year + 543).Substring(2) & "0" & ids
            ElseIf CountID = 3 Then
                id = "OR" & CStr(Now.Year + 543).Substring(2) & "00" & ids
            ElseIf CountID = 2 Then
                id = "OR" & CStr(Now.Year + 543).Substring(2) & "000" & ids
            ElseIf CountID = 1 Then
                id = "OR" & CStr(Now.Year + 543).Substring(2) & "0000" & ids
            End If
        End If
        SqlCon.Close()
        Return id

    End Function