Почему при получении токена, имя и пароль пользователя оказывается неверным?

На этой строке var response = client.PostAsync(APP_PATH + "/Token", content).Result; получаю response равным:

{StatusCode: 400, ReasonPhrase: 'Bad Request', Version: 1.1, Content: System.Net.Http.StreamContent, Headers:
{
  Pragma: no-cache
  X-SourceFiles: =?UTF-8?B?WjpceHh4XHByb2plY3R2aXN1YWxzdHVkaW8tQXJnaGhoTm90ZVxXZWJBcHBsaWNhdGlvbkFub25pbXVzMFxXZWJBcHBsaWNhdGlvbi5XRUJcVG9rZW4=?=
  Cache-Control: no-cache
  Date: Fri, 03 Mar 2017 16:40:05 GMT
  Server: Microsoft-IIS/10.0
  X-Powered-By: ASP.NET
  Content-Length: 138
  Content-Type: application/json; charset=UTF-8
  Expires: -1
}}

.

static Dictionary<string, string> GetTokenDictionary(string userName, string password)
        {
            var pairs = new List<KeyValuePair<string, string>>
            {
                new KeyValuePair<string, string>( "grant_type", "password" ),
                new KeyValuePair<string, string>( "username", userName ),
                new KeyValuePair<string, string> ( "Password", password )
            };
            var content = new FormUrlEncodedContent(pairs);
            using (var client = new HttpClient())
            {
                try
                {
                    var response =
                        client.PostAsync(APP_PATH + "/Token", content).Result;
                    var result = response.Content.ReadAsStringAsync().Result;
                    // Десериализация полученного JSON-объекта
                    Dictionary<string, string> tokenDictionary =
                        JsonConvert.DeserializeObject<Dictionary<string, string>>(result);
                    return tokenDictionary;

result равен:

"{\"error\":\"invalid_grant\",\"error_description\":\"Имя пользователя или пароль указаны неправильно.\"}"