Why can't I read an image from a byte array?

private URLResponse buildResponse(HttpURLConnection connection) throws IOException {
    int responseCode = connection.getResponseCode();

    HashMap<String, String> headers = new HashMap<>();
    for (Map.Entry<String, java.util.List<String>> headerEntry : connection.getHeaderFields().entrySet()) {
        headers.put(headerEntry.getKey(), String.join(",", headerEntry.getValue()));
    }

    ByteArrayOutputStream baos;
    if("45".contains(String.valueOf(responseCode).substring(0, 1))) {
        baos = readStream(connection.getErrorStream());
    }else{
        baos = readStream(connection.getInputStream());
    }
    return new URLResponse(responseCode, baos.toByteArray(), headers);
}

private ByteArrayOutputStream readStream(InputStream inputStream) throws IOException {
    ByteArrayOutputStream baos = new ByteArrayOutputStream();
    byte[] buff = new byte[4096];
    int len;
    while ((len = (byte) inputStream.read(buff)) > 0) {
        baos.write(buff, 0, len);
    }
    return baos;
}

public record URLResponse(int code, byte[] bytes, HashMap<String, String> headers) implements Serializable{
    @Serial
    private static final long serialVersionUID = 1L;
    public String response(){
        return new String(bytes);
    }
    public <T> T json(Class<T> clazz) {
        return Launch.gson.fromJson(response(), clazz);
    }
    public BufferedImage image() throws IOException {
        BufferedImage bi = ImageIO.read(new ByteArrayInputStream(bytes));
        if(bi == null){
            throw new NullPointerException("Image is null");
        }
        return bi;
    }
}