I have a problem, I want to convert an image from a url to base64, but I get an error

This is the url: <https://www.jotform.com/widget-uploads/imagePreview/92613577901663/2922362Principios%20-%20Resultados.png>

```
    public String ConvertImageURLToBase64(String url)
    {
        StringBuilder sb = new StringBuilder();

        Byte[] vbyte = this.GetImage(url);

        sb.Append(Convert.ToBase64String(vbyte, 0, vbyte.Length));

        return sb.ToString();
    }

    private byte[] GetImage(string url)
    {
        Stream stream = null;
        byte[] buf;

        try
        {
            WebProxy myProxy = new WebProxy();
            HttpWebRequest req = (HttpWebRequest)WebRequest.Create(url);

            HttpWebResponse response = (HttpWebResponse)req.GetResponse();
            stream = response.GetResponseStream();

            using (BinaryReader br = new BinaryReader(stream))
            {
                int len = (int)(response.ContentLength);
                buf = br.ReadBytes(len);
                br.Close();
            }

            stream.Close();
            response.Close();
        }
        catch (Exception exp)
        {
            buf = null;
        }

        return (buf);
    }

```