SecurityTokenRespone is null ?

IServiceManagement<IOrganizationService> serviceManagement = ServiceConfigurationFactory.CreateManagement<IOrganizationService>(new Uri("https://SERVICE_URI"));
AuthenticationCredentials authCreds = new AuthenticationCredentials();
authCreds.ClientCredentials.UserName.UserName = userName;
authCreds.ClientCredentials.UserName.Password = password;
AuthenticationCredentials tokenCreds = serviceManagement.Authenticate(authCreds);
SecurityTokenResponse securityToken = tokenCredentials.SecurityTokenResponse;