"Cannot create instance of type 'System.String' because it is missing a public parameterless constructor."

I am trying to implement the options-pattern and get the following error:

"Cannot create instance of type 'System.String' because it is missing a public parameterless constructor."

My setup is as this:

I have a Controller class like this:

public class MyController : Controller
{

  private readonly HttpClient _httpClient;
  private readonly MyConfig _myConfig;

  public MyController(HttpClient httpClient, IOptions<MyConfig> myConfig){
    _httpClient = httpClient;
    _myConfig = myConfig.Value;  <<<<<<<<<<<< FAILS HERE
  }

  [HttpGet]
  public async Task<IActionResult> Get(){
  ....
  var myHttpRequest = new HttpRequestMessage(HttpMethod.Get, _myConfig.MY_ENDPOINT);
  ...
  ...
  }
}

I have a JSON file - my appsettings that goes something like this:

{
"Tracer": {
"Trace": {
  "Default": "",
  "Smari": ""
  }
},

"MY_DB": "",
"MY_USER": "",
"MY_PW": "",
"MY_ENDPOINT": "https://my-test-site/offering",
"MY_CIGO": ""
}

And in my Startup.cs I've written the following:

services.Configure<MyConfig>(Configuration);

Lastly my MyConfig class that goes like this:

public class MyConfig 
{
    public string MY_ENDPOINT { get; set; }
}

Can anyone see, why it fails? I've checked my JSON file for syntax and it seems correct, so I am a bit lost.