I have a file `appsettings.json` that looks like this:

```
{
    "MyConfig": {
        "ConfigA": "value",
        "ConfigB": "value"
    }
}

```

In my `Startup.cs` I'm building my `IConfiguration`:

```
public ConfigurationRoot Configuration { get; set; }

public Startup(ILoggerFactory loggerFactory, IHostingEnvironment environment)
{
      var builder = new ConfigurationBuilder()
                     .SetBasePath(environment.ContentRootPath)
                     .AddJsonFile("appsettings.json", optional: false, reloadOnChange: true)                             
                     .AddEnvironmentVariables();

      Configuration = builder.Build();
}

public void ConfigureServices(IServiceCollection services)
{
      //GetSection returns null...
      services.Configure<MyConfig>(Configuration.GetSection("MyConfig"));
}

```

But `Configuration.GetSection("MyConfig")` always returns `null`, although the value exists in my JSON file. `Configuration.GetSection("MyConfig:ConfigA")` works just fine.

What am I doing wrong?