Entity Framework Code First - Eager Loading not working as expected?

I have the following Entity Framework POCO classes:

public class Customer
{
    public int Id {get;set;}
    public string Name {get;set;}

    public virtual ICollection<Order> Orders {get;set;}
}

public class Order
{
    public int Id {get;set;} 
    public int CustomerId {get;set;}
    public int OrderTypeId {get;set;}

    public virtual OrderType Type {get;set;}
    public virtual Customer Customer {get;set;}
} 

public class OrderType 
{
    public int Id {get;set;}
    public virtual ICollection<Order> Orders {get;set;}
}

The problem is that when I return my ICollection I'm getting the Orders alright, but the OrderType property of the Order is not being populated. My Order will contain the following detail:

Id:          1
CustomerId:  1
Customer:    Populated
OrderTypeId: 3
Type:        null        // Never returned

My mapping code looks like the following:

public void ConfigureOrder(ModelBuilder builder)
{
    // Mapping from Order -> Customer
    builder.Entity<Order>()
        .HasRequired(x => x.Customer)
            .WithMany(x => x.Orders)
                .HasConstraint((order, cust) => order.CustomerId == cust.Id);

    // Mapping from Order -> OrderType
    builder.Entity<Order>()
        .HasRequired(x => x.OrderType)
            .WithMany(x => x.Orders)
                .HasConstraint((order, type) => order.OrderTypeId == type.Id);
}

I've then disabled Lazy loading in my context:

public Context(string connectionString)
    : base(connectionString)
{
    ObjectContext.ContextOptions.LazyLoadingEnabled = false;
}

So to return the data in my repository I use the Include method of System.Data.Entity:

var query = from item in context.Customers
                .Include(x=> x.Orders)
            where item.Id == customerId
            select item;

I was assuming that because I couldn't specify Orders.OrderType, that was the problem, so I tried some variations:

1 -> .Include(x=> x.Orders.FirstOrDefault().OrderType)
2 -> .Include("Orders")
3 -> .Include("Orders")
     .Include("Orders.OrderType")

But I can never get the OrderType property to be returned, unless I just load the Order directly:

var query = from item in context.Orders
                .Include(x=> x.OrderType)
            select item;

This code will correctly return the OrderType within the order.