tx, err := db.Begin()
if err != nil {
    return err
}
defer tx.Rollback()

var results []MyStruct
qSelect := tx.Prepare(select_sql)
qInsert := tx.Prepare(insert_sql)

for _, thing := range *things {
    var id int
    var err error
    var result sql.Result
    err = qSelect.QueryRow(thing.FieldOne).Scan(&id)
    if err != nil {
        if err == sql.ErrNoRows {
            result, err = qInsert.Exec(thing.FieldOne, thing.FieldTwo)
            if err != nil { /* this is the line where I get the error */
                return err
            }
        } else {
            return err
        }
    }
    if num, err := result.RowsAffected(); err != nil {
        return err
    } else if num > 0 {
        results = append(results, thing)
    }
}

tx.Commit()