how to find out ExecuteScalar() is dbnull

ExecuteScalar() have no data (if statement) not working ......

Dim cmdCode As New OracleCommand

        cmdCode.Connection = sgcnn

        cmdCode.CommandText = "select max(SUBSTR(client_code,4,7)) from CLIENT_INFO where (client_code Like '%' || :SysUser || '%') "

        cmdCode.Parameters.Add("@SysUser", OracleDbType.Varchar2).Value = strMessage

'Dim sqlResult As Object = cmdCode.ExecuteScalar()

        'If sqlResult Is Nothing Then

        '    ' No result found

        '    strMessage.Append("0001")

        '    txtClientCode.Text = strMessage.ToString

        'Else

        '    'result found

        '    Dim newProdID As Int32 = 0

        '    newProdID = Convert.ToInt32(cmdCode.ExecuteScalar())

        '    newProdID = newProdID + 1

        '    strMessage.Append(newProdID.ToString("0000"))

        '    txtClientCode.Text = strMessage.ToString

        'End If

plz tell me what to do ......