Go で Base64 イメージをデコードすると「不明なイメージ形式」エラーが発生するのはなぜですか?

import _ "image/png"

input := "data:image/png;base64,iVkhdfjdAjdfirtn="
b64data := input[strings.IndexByte(input, ',')+1:]

package main

import (
    "fmt"
    "image/png"
    "log"
    "os"
    "strings"

    "github.com/nfnt/resize"
)

func main() {
    dataURL := os.Getenv("IMAGE_DATA_URL")

    if dataURL == "" {
        log.Fatal("Missing IMAGE_DATA_URL environment variable")
    }

    // Remove Data URI scheme prefix
    b64data := dataURL[strings.IndexByte(dataURL, ',')+1:]

    // Decode base64 string
    reader := base64.NewDecoder(base64.StdEncoding, strings.NewReader(b64data))

    // Register PNG format handler and decode the image config
    _ = png.RegisterFormat("png", "png", png.PNGConfig{})
    config, _, err := image.DecodeConfig(reader)
    if err != nil {
        log.Fatal(err)
    }

    // Resize the image
    newSize := resize.Thumbnail(0, 0, config.Width, config.Height)
    scaledImage := resize.Resize(uint(newSize.Width), uint(newSize.Height), reader, resize.Lanczos3)

    // Save the resized image
    outFile, err := os.Create("resized-image.png")
    if err != nil {
        log.Fatal(err)
    }

    err = png.Encode(outFile, scaledImage)
    if err != nil {
        log.Fatal(err)
    }

    fmt.Printf("Image saved to: %s", "resized-image.png")
}