Validate ISBN-13 Numbers in Golang

package main

import (
    "fmt"
    "strconv"
)

func sum13(isbn string) (int, error) {
    s := 0
    w := 1
    for _, v := range isbn {
        n, err := strconv.Atoi(string(v))
        if err != nil {
            return -1, fmt.Errorf("Failed to convert ISBN-13 character to int: %s", string(v))
        }
        s += n * w
        if w == 1 {
            w = 3
        } else {
            w = 1
        }
    }
    return s, nil
}

func validISBN13(isbn13 string) (string, error) {
    if len(isbn13) != 12 && len(isbn13) != 13 {
        return "", fmt.Errorf("A string of length 12 or 13 is required to calculate the ISBN-13 check digit. Provided was: %s", isbn13)
    }
    s, err := sum13(isbn13[:12])
    if err != nil {
        return "", err
    }
    d := 10 - (s % 10)
    if d == 10 {
        return "0", nil
    }
    return strconv.Itoa(d), nil
}

func main() {

    str := "1112223339123"
    _, err := validISBN13(str)
    if err != nil {
        fmt.Println("InValid")
    } else {
        fmt.Println("Valid")
    }

}    

Output

Valid