Requesting an access token using JWT C# httpClient Response returns "the input is not a valid base-64 string ..."

Question

Please see codes below. I got the following error on line

var result = response.Content.ReadAsStringAsync().GetAwaiter().GetResult();

Error: The input is not a valid Base-64 string as it contains a non-base 64 character, more than two padding characters, or an illegal character among the padding characters.

Not sure what i did wrong. Please help!

using System;
using System.Collections.Generic;
using System.IO;
using System.Threading.Tasks;
using System.IdentityModel.Tokens;
using System.Security.Cryptography;
using Org.BouncyCastle.OpenSsl;
using Org.BouncyCastle.Security;
using Org.BouncyCastle.Crypto;
using Org.BouncyCastle.Crypto.Parameters;
using System.Net;
using Newtonsoft.Json;
using System.Net.Http;
using System.Net.Http.Headers;

namespace ConsoleApp1
{
    class Program
    {
        static void Main(string[] args)
        {
            string privateKeyFilePath = "C:\\Users\\privateKey.txt";
            string privateKey;
            string clientId = "myclientid";
            string serraviewUri = "https://instance.serraview.com";

            try
            {
                privateKey = File.ReadAllText(privateKeyFilePath);
            }
            catch (Exception ex)
            {
                return;
            }

            // Convert PEM to RSA using the BouncyCastle library.
            var rsa = new RSACryptoServiceProvider();
            RSAParameters rsaParameters;

            using (var privateKeyReader = new StringReader(privateKey))
            {
                var pemReader = new PemReader(privateKeyReader);
                var keyPair = (AsymmetricCipherKeyPair)pemReader.ReadObject();
                var privKey = (RsaPrivateCrtKeyParameters)keyPair.Private;
                rsaParameters = DotNetUtilities.ToRSAParameters(privKey);
            }
            rsa.ImportParameters(rsaParameters);

            // The only signing algorithm supported by the Serraview Authorization Server is RSA using SHA-256 hashing algorithm.
            var signingKey = new RsaSecurityKey(rsa);
            var signingCredentials = new SigningCredentials(signingKey, SecurityAlgorithms.RsaSha256Signature, SecurityAlgorithms.Sha256Digest);

            // Get audience for the JWT. This will be the url for the authorization server we request an access token from.
            Uri uri;
            Uri.TryCreate(serraviewUri, UriKind.Absolute, out uri);

            Uri audience;
            if (!Uri.TryCreate(uri, "/oauth2/token", out audience))
            {
                return;
            }

            // Build JWT
            var jwt = new JwtSecurityToken(
                clientId,
                audience.ToString(),
                null, 
                DateTime.UtcNow /*not before*/,
                DateTime.UtcNow.AddMinutes(5) /*expires*/,
                signingCredentials);

            // Serialize our jwt object to an actual JWT string
            var jwtString = new JwtSecurityTokenHandler().WriteToken(jwt);

            // Request an access token
            // Serraview servers require TLS1.2
            ServicePointManager.SecurityProtocol = SecurityProtocolType.Ssl3 | SecurityProtocolType.Tls | SecurityProtocolType.Tls11 | SecurityProtocolType.Tls12;

            string accessToken;
            using (var client = new System.Net.Http.HttpClient())
            {
                client.BaseAddress = uri;

                var content = new System.Net.Http.FormUrlEncodedContent(
                new[]
                {
                    new KeyValuePair<string, string>("grant_type","urn:ietf:params:oauth:grant-type:jwt-bearer"),
                    new KeyValuePair<string, string>("assertion", jwtString)
                });

                var response = client.PostAsync("/oauth2/token", content).GetAwaiter().GetResult();
                var result = response.Content.ReadAsStringAsync().GetAwaiter().GetResult();
                dynamic parsedJson = JsonConvert.DeserializeObject(result);
                accessToken = parsedJson["access_token"];
            }
            
            Console.WriteLine(accessToken);
            Console.Read();  
        }

    }
}

I tried webclient and the same issue. The response returns 200 but output "Error: the input...." after encoding it to a string.

The following is part of the SerraView API doc.

I have tested with Postman and it threw "unsupported grant_type"