Passing form data to controller method - child properties are not getting filled in

Question

My app is using JavascriptMVC on the client side, and ASP MVC is basically functioning only as a REST service. Here's a typical controller method:

public JsonResult Update(CustomerDto dto)
{
  var repository = Factory.NewCustomerRepository())
  // ... Convert DTO back to entity and save changes
  return Json(dto);
}

The problem is, my CustomerDTO contains some properties that aren't getting converted from the form data into the objects that they should be. For example, PhoneNumbers:

public class CustomerDto
{
    public int Id { get; set; }
    public string FirstName { get; set; }
    public string MiddleName { get; set; }
    public string LastName { get; set; }
    public PhoneNumberDto[] PhoneNumbers { get; set; }
    // ... more properties
}
public class PhoneNumberDTO
{
    public int Id { get; set; }
    public int CustomerId { get; set; }
    public string Label { get; set; }
    public string Number { get; set; }
}

In the controller action, PhoneNumbers will have the correct number of elements in the array, but each object will have only null/default values. I've verified that the request is sending all of the appropriate form data:

Id          26
FirstName   A
LastName    Person
MiddleName  Test
PhoneNumbers[0][CustomerID  26
PhoneNumbers[0][Id] 5
PhoneNumbers[0][Label]  Mobile
PhoneNumbers[0][Number] (555)555-5555
PhoneNumbers[1][CustomerID  26
PhoneNumbers[1][Id] 8
PhoneNumbers[1][Label]  Home
PhoneNumbers[1][Number] (654)654-6546

Any ideas on what could be going on? Am I just wrong in thinking that MVC3 can map nested objects from the form values automatically? Thanks for any help!