Subclassing IdentityUser ASP.NET - asp.net

Question

I have a Web API and I want my AutoHubUser to be used instead of the default one (IdentityUser). But I keep getting internal server error when I try to save it. What needs to be changed?
public class AutoHubUser : IdentityUser
{
[Required]
[DataType(DataType.EmailAddress)]
public string Email { get; set; }
[Required]
[DataType(DataType.Date)]
[Display(Name = "Birth Date")]
public DateTime BirthDate { get; set; }
[Required]
public Gender Gender { get; set; }
public virtual ICollection<Auto> Autos { get; set; }
public virtual ICollection<UserMilestone> UserMilestones { get; set; }
public virtual ICollection<Friendship> Friendships { get; set; }
}
The account controller
// POST api/Account/Register
[AllowAnonymous]
[Route("Register")]
public async Task<IHttpActionResult> Register(RegisterBindingModel model)
{
if (!ModelState.IsValid)
{
return BadRequest(ModelState);
}
IdentityUser user = new AutoHubUser
{
UserName = model.UserName,
Email = model.Email,
BirthDate = model.BirthDate,
Gender = model.Gender
};
IdentityResult result = await UserManager.CreateAsync(user, model.Password);
//IdentityResult result = await new UserManager<AutoHubUser>().CreateAsync(user, model.Password);
IHttpActionResult errorResult = GetErrorResult(result);
if (errorResult != null)
{
return errorResult;
}
return Ok();
}
I also modified the RegisterBindingModel
public class RegisterBindingModel
{
[Required]
[Display(Name = "User name")]
public string UserName { get; set; }
[Required]
[StringLength(100, ErrorMessage = "The {0} must be at least {2} characters long.", MinimumLength = 6)]
[DataType(DataType.Password)]
[Display(Name = "Password")]
public string Password { get; set; }
[DataType(DataType.Password)]
[Display(Name = "Confirm password")]
[Compare("Password", ErrorMessage = "The password and confirmation password do not match.")]
public string ConfirmPassword { get; set; }
[Required]
[DataType(DataType.EmailAddress)]
public string Email { get; set; }
[Required]
[DataType(DataType.Date)]
public DateTime BirthDate { get; set; }
[Required]
public Gender Gender { get; set; }
}
What else needs to be modified? It must be in the UserManager's CreateAsync method. But I haven't been able to find an example online. Thanks!