WebApi returns empty JSON instead of serialized object

Question

So simple yet I know not why it fails. In a WebAPI 2.0 ASP.NET MVC (pre-core) controller method, I have this:

[Route("GetItem")]
[HttpGet]
public ItemVM GetItem() {
    var item = new ItemVM();  // Constructor initializes
    return item;
}

When I run the code, the debugger shows this in item:

item          {ViewModels.ItemVMs.ItemVM}
    firstItem {ViewModels.ItemVMs.FirstItemVM}
        id       0
        archived false
        name     null

Yet WebAPI returns only this:

{}

I have tried suggestions like Newtonsoft json serializer returns empty object but Visual Studio 2017 says CreateProperties does not exist to override.

Any help would be much appreciated.

EDIT:

LOL I told you it is a simple answer. Here's the class:

public class ItemVM {
    FirstItemVM firstItem { get; set; }
    
    public ItemVM() {
        this.firstItem = FirstItemVM.ToMap( new Entities.Item());
    }
}

public class FirstItemVM {
    public int id { get; set; }
    public bool archived { get; set; }
    public string name { get; set; }

    public static readonly Expression<Func<Entities.FirstItem, FirstItemVM>> 
        Map (e) => new FirstItemVM {
            id = e.id,
            archived = e.archived,
            name = e.name
    };
    public static readonly Func<Entities.FirstItem, FirstItemVM>
        ToMap = FirstItemVM.Map.Compile();
}

Answer 1

Most likely ItemVM.firstItem is not public (for example, internal), and JSON serializer will only serialize public properties by default (unless you non-public property explicitly to be serialized).