Google OAuth2 Service Account Access Token Request gives 'Invalid Request' Response

I'm trying to communicate with my app's enabled BigQuery API via the server to server method.

I've ticked all the boxes on this Google guide for constructing my JWT as best I can in C#.

And I've Base64Url encoded everything that was necessary.

However, the only response I get from google is a 400 Bad Request

"error" : "invalid_request"

I've made sure of all of the following from these other SO questions:

• The signature is properly encrypted using RSA and SHA256
• I am using POST and using application/x-www-form-urlencoded content type
• Escaped all the backslashes in the claim set
• Tried various grant_type and assertion values in the POST data

I get the same result when I use Fiddler. The error message is frustratingly lacking in detail! What else can I try?! Here's my code:

class Program
{
    static void Main(string[] args)
    {
        // certificate
        var certificate = new X509Certificate2(@"<Path to my certificate>.p12", "notasecret");

        // header
        var header = new { typ = "JWT", alg = "RS256" };

        // claimset
        var times = GetExpiryAndIssueDate();
        var claimset = new
        {
            iss = "<email address of the client id of my app>",
            scope = "https://www.googleapis.com/auth/bigquery",
            aud = "https://accounts.google.com/o/oauth2/token",
            iat = times[0],
            exp = times[1],
        };

        // encoded header
        var headerSerialized = JsonConvert.SerializeObject(header);
        var headerBytes = Encoding.UTF8.GetBytes(headerSerialized);
        var headerEncoded = Base64UrlEncode(headerBytes);

        // encoded claimset
        var claimsetSerialized = JsonConvert.SerializeObject(claimset);
        var claimsetBytes = Encoding.UTF8.GetBytes(claimsetSerialized);
        var claimsetEncoded = Base64UrlEncode(claimsetBytes);

        // input
        var input = headerEncoded + "." + claimsetEncoded;
        var inputBytes = Encoding.UTF8.GetBytes(input);

        // signiture
        var rsa = certificate.PrivateKey as RSACryptoServiceProvider;
        var cspParam = new CspParameters
        {
            KeyContainerName = rsa.CspKeyContainerInfo.KeyContainerName,
            KeyNumber = rsa.CspKeyContainerInfo.KeyNumber == KeyNumber.Exchange ? 1 : 2
        };
        var aescsp = new RSACryptoServiceProvider(cspParam) { PersistKeyInCsp = false };
        var signatureBytes = aescsp.SignData(inputBytes, "SHA256");
        var signatureEncoded = Base64UrlEncode(signatureBytes);

        // jwt
        var jwt = headerEncoded + "." + claimsetEncoded + "." + signatureEncoded;

        Console.WriteLine(jwt);

        var client = new HttpClient();
        var uri = "https://accounts.google.com/o/oauth2/token";
        var post = new Dictionary<string, string>
        {
            {"assertion", jwt},
            {"grant_type", "urn%3Aietf%3Aparams%3Aoauth%3Agrant-type%3Ajwt-bearer"}
        };
        var content = new FormUrlEncodedContent(post);
        var result = client.PostAsync(uri, content).Result;

        Console.WriteLine(result);
        Console.WriteLine(result.Content.ReadAsStringAsync().Result);
        Console.ReadLine();
    }

    private static int[] GetExpiryAndIssueDate()
    {
        var utc0 = new DateTime(1970, 1, 1, 0, 0, 0, 0, DateTimeKind.Utc);
        var issueTime = DateTime.Now;

        var iat = (int)issueTime.Subtract(utc0).TotalSeconds;
        var exp = (int)issueTime.AddMinutes(55).Subtract(utc0).TotalSeconds;

        return new[]{iat, exp};
    }

    private static string Base64UrlEncode(byte[] input)
    {
        var output = Convert.ToBase64String(input);
        output = output.Split('=')[0]; // Remove any trailing '='s
        output = output.Replace('+', '-'); // 62nd char of encoding
        output = output.Replace('/', '_'); // 63rd char of encoding
        return output;
    }
}