Instagram: почему при получении access_token происходит 400 BadRequest?

var body = new List<KeyValuePair<string, string>>
                {
                    new KeyValuePair<string, string>("grant_type", "authorization_code"),
                    new KeyValuePair<string, string>("code", code),
                    new KeyValuePair<string, string>("redirect_uri", redirectUri),
                    new KeyValuePair<string, string>("client_id", Options.ClientId),
                    new KeyValuePair<string, string>("client_secret", Options.ClientSecret)
                };

// Request the token
                var tokenResponse =
                    await _httpClient.PostAsync(TokenEndpoint, new FormUrlEncodedContent(body));