Recaptcha v2 always returns 'missing-input-secret'

I use Blogengine and want to add Recaptcha v2 from Billkrat. In RecaptchaValidator.cs we find the following code:

public RecaptchaResponse Validate()
{
    CheckNotNull(this.PrivateKey, "PrivateKey");
    CheckNotNull(this.RemoteIP, "RemoteIp");
    CheckNotNull(this.Challenge, "Challenge");
    CheckNotNull(this.Response, "Response");

    if (this.Challenge == string.Empty || this.Response == string.Empty)
    {
        return RecaptchaResponse.InvalidSolution;
    }

    var request = (HttpWebRequest)WebRequest.Create(VerifyUrl);

    // to avoid issues with Expect headers
    request.ProtocolVersion = HttpVersion.Version10;
    request.Timeout = 30 * 1000 /* 30 seconds */;
    request.Method = "POST";
    request.UserAgent = "reCAPTCHA/ASP.NET";
    request.ContentType = "application/x-www-form-urlencoded";

    var formdata = String.Format(
        HttpUtility.UrlEncode(this.PrivateKey),
        HttpUtility.UrlEncode(this.Response),
        HttpUtility.UrlEncode(this.RemoteIP));

    var formbytes = Encoding.ASCII.GetBytes(formdata);
    try
    {
        using (var requestStream = request.GetRequestStream())
        {
            requestStream.Write(formbytes, 0, formbytes.Length);
        }

        dynamic results;

        using (var httpResponse = request.GetResponse())
        {
            var httpResponseStream = httpResponse.GetResponseStream();
            if (httpResponseStream == null)
            {
                return RecaptchaResponse.RecaptchaNotReachable;
            }

            using (TextReader readStream = new StreamReader(httpResponseStream, Encoding.UTF8))
            {
                results = JObject.Parse(readStream.ReadToEnd());
            }
        }

And Google always returns the error 'missing-input-secret'. Any hint is appreciated.