ASP.NET 5, MVC6, WebAPI -> ModelState.IsValid always returns true

I've seen a lot of posts about IsValid always being true but none of them have helped me solve this problem. I'm also seeing this problem in ASP.NET 4 using MVC5. So clearly I'm missing a step somewhere.

Controller method:

public IHttpActionResult Post([FromBody]ValuesObject value)
{
    if (ModelState.IsValid)
    {
        return Json(value);
    }
    else
    {
        return Json(ModelState);
    }
}

ValuesObject Class:

public class ValuesObject
{
    [Required]
    public string Name;

    [Range(10, 100, ErrorMessage = "This isn't right")]
    public int Age;
}

Body of the POST:

{
  Age: 1
}

ModelState.IsValid is true.

But I would expect both the Required and Range validations to fail.

What am I missing??

Thanks,

Kevin