ASP.NET MVC API model not parsing

I am having some problems parsing my model in ASP.NET MVC API

This is my API controller:

public class UserController : ApiController
{
    // Hent liste af personer
    public IEnumerable<UserModel> Get()
    {
        return new UserModel[] { new UserModel(), new UserModel() };
    }

    // Hente enkelt person
    public UserModel Get(int id)
    {
        return new UserModel();
    }

    // Opret person
    [ValidationActionFilter]
    public CreateUserRespose Post([FromBody]UserModel model)
    {
        CreateUserRespose rs = new CreateUserRespose();
        return rs;
    }

    // Rediger person
    public UserModel Put(int id, [FromBody]UserModel model)
    {
        return new UserModel();
    }

    // Slet person
    public UserModel Delete(int id)
    {
        return new UserModel();
    }
}

}

And the UserModel:

public class UserModel
{
    [Required]
    [StringLength(500)]
    public String FristName { get; set; }
    [Required]
    [StringLength(500)]
    public String LastName { get; set; }
    [Required]
    [StringLength(250)]
    public String Email { get; set; }
    [Required]
    public String MatrikelId { get; set; }
}

When I call though Fiddler to the Post command with the following body

FirstName=Fistname MiddleName&LastName=SomeName&[email protected]&MatrikelId=1234

Will the action Post be called, but the model is null, and ModelState.IsValid is true, the same happens if I send no data with the body! What am I doing wrong here?

Update: I have tryed sending the data as json instead Fiddler:

User-Agent: Fiddler
Host: localhost:51268
Content-Length: 102
Content-type: application/json

{"FristName":"Kasper asdasd","LastName":"asdasdasd","Email":"[email protected]","MatrikelId":"132456asd"}

But should the model state not be invalid when the model is null?