appsettings.json in ASP.net Core 2.0 Preview configuration GetSection null

I was trying to call GetSection from injected configuration in the Startup.cs. The Value was null, while indexer to a concrete section value returns non-null value. It seems to me a bug behind the GetSection method or I am wrong with it?

appsettings.json:

{ "MyConfig": { "ConfigA": "valueA", "ConfigB": "valueB" } }

Program.cs:

    public static void Main(string[] args)
    {
        var host = BuildWebHost(args);
        host.Run();
    }

    public static IWebHost BuildWebHost(string[] args) =>
        WebHost.CreateDefaultBuilder(args)
            .UseStartup<Startup>()
            .Build();

Startup.cs:

public class Startup
{
    public Startup(IConfiguration configuration)
    {
        Configuration = configuration;
    }

    public IConfiguration Configuration { get; }

    // This method gets called by the runtime. Use this method to add services to the container.
    public void ConfigureServices(IServiceCollection services)
    {
        services.AddMvc();
    }

    // This method gets called by the runtime. Use this method to configure the HTTP request pipeline.
    public void Configure(IApplicationBuilder app, IHostingEnvironment env)
    {
        var mySection = this.Configuration.GetSection("MyConfig");

        var myVal = this.Configuration["MyConfig:ConfigA"];