Validate ISBN-10 Numbers in Golang

package main

import (
    "fmt"
    "strconv"
)

func sum10(isbn string) (int, error) {
    s := 0
    w := 10
    for k, v := range isbn {
        if k == 9 && v == 88 {
            s += 10
        } else {
            n, err := strconv.Atoi(string(v))
            if err != nil {
                return -1, fmt.Errorf("Failed to convert ISBN-10 character to int: %s", string(v))
            }
            s += n * w
        }
        w--
    }
    return s, nil
}

func validISBN10(isbn10 string) (string, error) {
    if len(isbn10) != 9 && len(isbn10) != 10 {
        return "", fmt.Errorf("A string of length 9 or 10 is required to calculate the ISBN-10 check digit. Provided was: %s", isbn10)
    }
    s, err := sum10(isbn10[:9])
    if err != nil {
        return "", err
    }
    d := (11 - (s % 11)) % 11
    if d == 10 {
        return "X", nil
    }
    return strconv.Itoa(d), nil
}

func main() {

    str := "1112223339"
    _, err := validISBN10(str)
    if err != nil {
        fmt.Println("InValid")
    } else {
        fmt.Println("Valid")
    }

}    

Output

Valid