MVC :: Model Contain No Data When Postings Form?

I have two model class

Class CustomerMpdel
{
public string Firstname {get;set;}
public string Lastaname {get set;}
}
Class UserModel
{
public int UserID {get; set;}
pulic Customer Customer{get; set;}
}
<% Using(HTMl.BeginForm()) {%>
<%:Html.TextboxFor(a=>a.UserID)%>
<%:Html.TextboxFor(a=>a.Customer.Firstname)%>
<%:Html.TextboxFor(a=>a.Customer.Lastname)%>
<input type="submt"/>
<%}%>
public ActionResult (UserModel user)
{
///gettig user parameter null here
return view();
}