C# – ModelState is valid with null model

I have a Model object with a required attribute

public class ApiPing
{
    [Required]
    public DateTime ClientTime { get; set; }

    public DateTime ServerTime { get; set; }
}

I have a Controller method that checks model state.

public IHttpActionResult Ping(ApiPing model)
{    
    if (!ModelState.IsValid)
        return BadRequest(ModelState);

    model.ServerTime = DateTime.UtcNow;

    return Ok(model);
}

If I submit a submit a proper request (with a model) to the action method I get an correct value from ModeState.IsValid (true). However, when I submit an invalid request (without a model, so the model is null) I get an erroneous ModelState.IsValid (also true).

I could simply check if the model is null in my code, but that smells. Is this an intended 'feature' or a bug in ModelState validation? Am I doing something wrong ? Am I expecting too much ?