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 sqlConn As New SqlConnection(ConfigurationSettings.AppSettings("connectionstring"))
'Determine order id number...
Dim order_id As Integer
Dim strSQL As String
strSQL = "Select MAX(order_id) from mkt_order"
Dim sqlCmd As New SqlCommand(strSQL, sqlConn)
Dim sqlDA As New SqlDataAdapter(sqlCmd)
Dim sqlDS As New DataSet
sqlDA.Fill(sqlDS, "item")
If sqlDS.Tables(0).Rows.Count <> 0 Then
order_id = Convert.ToInt32(sqlDS.Tables(0).Rows(0)(0).ToString()) + 1
Else
order_id = 1
End If