what is wrong with this code?

field="id";
table="MInvMonth";
condition="machine_id=37";
 public static String getConditionedField(String field, String table, String condition)
    try
    {
        if (cmd == null) getConnection();
        cmd.CommandText = "Select " + field + " from " + table + " where " + condition;
        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 "".