Determine Next Available Order ID

I am trying to determine the next available order id using the method below. It works provided the table has a record in it. If it doesn't I get the error "Input string was not in a correct format." I am certain that it is because the query is returning a value of NULL. How can get around that or check for the NULL value?

' Establish data connection...

Dim sqlConnAsNew SqlConnection(ConfigurationSettings.AppSettings("connectionstring"))

'Determine order id number...

Dim order_idAsInteger

Dim strSQLAsString

strSQL = "Select MAX(order_id) from mkt_order"

Dim sqlCmdAsNew SqlCommand(strSQL, sqlConn)

Dim sqlDAAsNew SqlDataAdapter(sqlCmd)

Dim sqlDSAsNew DataSet

sqlDA.Fill(sqlDS, "item")

If sqlDS.Tables(0).Rows.Count <> 0Then

order_id = Convert.ToInt32(sqlDS.Tables(0).Rows(0)(0).ToString()) + 1

Else

order_id = 1

EndIf

This is because a DBNull will be returned if you use MAX() function and there is no (qualified) record in the table. Let's try TOP...ORDER BY... to avoid using MAX():

strSQL = "Select TOP 1 order_id from mkt_order ORDER BY order_id DESC"