what is wrong with this code?

try
{
    if (cmd == null) getConnection();
    cmd.CommandText = "Select id from HMonthInventary where machine_id=37";
    SQLiteDataReader reader = cmd.ExecuteReader();
    if (reader.HasRows==true)
    {
        reader.Read();
        string s = reader[0].ToString(); // return first element
        reader.Close();
        return s;
    }
    reader.Close();
    return null;
}
catch (Exception e)
{
    MessageBox.Show("Caught exception: " + e.Message+"|"+cmd.CommandText);
    return null;
}

I checked the sql statement, it turns the right value. why can't I read it? the returnvalue is "".