为什么我的hmac密钥关闭了？

I am trying to generate an hmac key for use in an image proxy. I have:

https://play.golang.org/p/fec_N2Nim4

package main

import (
    "crypto/hmac"
    "crypto/sha256"
    "encoding/base64"
    "fmt"
)

func main() {
    mac := hmac.New(sha256.New, []byte("secret key"))
        mac.Write([]byte("https://octodex.github.com/images/codercat.jpg"))
        want := mac.Sum(nil)
        fmt.Println("result: ",base64.URLEncoding.EncodeToString(want)) // expect "sXyMwWKIC5JPCtlYOQ2f4yMBTqpjtUsfI67Sp7huXIYY="
}

This is based off the package's recommended formula for generating the key as well as other methods I have tried, which all result in the same key. The url given in the example, however, has an "s" prepended to the front:

http://localhost:8080/500,sXyMwWKIC5JPCtlYOQ2f4yMBTqpjtUsfI67Sp7huXIYY=/https://octodex.github.com/images/codercat.jpg

vs what I am getting:

http://localhost:8080/500,XyMwWKIC5JPCtlYOQ2f4yMBTqpjtUsfI67Sp7huXIYY=/https://octodex.github.com/images/codercat.jpg

As per the docs of the given library, the key format is as below:

s{signature}

Please check the docs at Url signing signature