C# 对2captcha使用POST方法时出错

我想用2captchaapi解决验证码问题

string Key = "12345678901234567890123456789012";
string Method = "base64";

string DataForPost = $"key={Key}&method={Method}&imginstructions={Imginstructions}";
request.ContentLength = DataForPost.Length;

string responseText = "";

byte[] bytes = Encoding.ASCII.GetBytes(DataForPost);
request.ContentLength = bytes.Length;

using (Stream reqStream = request.GetRequestStream())
{
    reqStream.Write(bytes, 0, bytes.Length);
    reqStream.Close();
}

using (WebResponse resp = request.GetResponse())
{
    Stream respStream = resp.GetResponseStream();
    using (StreamReader sr = new StreamReader(respStream))
    {
        responseText = sr.ReadToEnd();
    }
}                

Imginstruction是图像的base64字符串，大小约为4KB。当我运行上述代码时，我收到了错误消息——“error\u ZERO\u CAPTCHA\u FILESIZE”。这个错误的意思是“文件大小小于100字节”，但我无法理解，因为文件大小约为4kb

如果你对这个问题有什么想法，请告诉我