Por que este error en el Execute.Scalar()?

[kutulay]

Esta es la consulta

private void button1_Click(object sender, EventArgs e)
{
Codigo = textBox2.Text.Trim();
cmd = new MySqlCommand();
cmd.Connection = conn;
cmd.CommandType = CommandType.Text;
cmd.CommandText = "select unds from '+codigo+'";



conn.Open();

Object Valor = null;
Valor = cmd.ExecuteScalar();

if(Valor.ToString().Trim() != "")
{
if (resultado >= int.Parse(Valor.ToString().Trim()))
{
}
}

}


}
}