The required column was not present in the results of a 'FromSql' operation

I've just starting learning MVC6 with EF7. I have a stored proc that I'd like to return a portion of the fields that are in my model. If I don't return every field in my model, I'm getting "The required column 'FirstName' was not present in the results of a 'FromSql' operation".

Is there a way to get make some columns not required so I can return just a portion of the fields in my model?

model:

public class LoginViewModel
{
    [Key]
    public int UserID { get; set; }

    [Required]
    [Display(Name = "Username")]
    public string Username { get; set; }

    [Required]
    [DataType(DataType.Password)]
    [Display(Name = "Password")]
    public string Password { get; set; }

    [Required]
    [DataType(DataType.Password)]
    [Display(Name = "Protected ID")]
    public string ProtectedID { get; set; }

    public string FirstName { get; set; }
    public string LastName { get; set; }
}

My proc for testing:

CREATE PROCEDURE [dbo].[aaa_TopXXUsersTest]
@NumToReturn int = 10
AS
BEGIN
    SET NOCOUNT ON;
    select top(@NumToReturn) UserID, LastName, Username,Password, ProtectedID from Users where Deleted = 0

END

and last, my controller code:

public IActionResult Index()
{
    var user = _context.Set<LoginViewModel>().FromSql("dbo.aaa_TopXXUsersTest @NumToReturn = {0}", 20);

    return View(user);
}

If I include all the fields of my model in my stored proc the call work fine, but I can't seem to return just a subset. Is there a way to make some of the fields not required?