Representing a many-to-many relationship with lookup table in Entity Framework

Question

I have a SQL database (which isn't normalized and doesn't have primary keys, only indexes which I cannot change the schema of) that has a many-to-many relationship.

Reservations can have many Customers and vice-versa. The Reservation table is keyed off ReservationID and TransactionLineNumber. The Customer table has IPCode as its key.

There is a lookup table that is used to create this relationship that has the three keys and nothing else.

Below is my DbContext configuration code:

        modelBuilder.Entity<ReservationDetail>()
            .HasMany(rd => rd.LinkedCustomers)
            .WithMany(c => c.Reservations)
            .Map(m =>
            {
                m.ToTable("r_transaction_detail_ip");
                m.MapLeftKey("reservation_id", "transaction_line_number");
                m.MapRightKey("ip_number");
            });

Here are snippets of the classes for each, and their entity configurations:

Customer

public class Customer
{
    public string IpNumber { get; set; }

    public string CustomerName { get; set; }

    public virtual ICollection<ReservationDetail> Reservations { get; set; }
}

        ToTable("ip");
        HasKey(c => c.IpNumber);

        Property(c => c.IpNumber).HasColumnName("ipcode");
        Property(c => c.CustomerName).HasColumnName("displayname");

Reservation

public class ReservationDetail
{
    public string ReservationId { get; set; }

    public short TransactionLineNumber { get; set; }

    ...

    public virtual ICollection<Customer> LinkedCustomers { get; set; } 
}

        ToTable("r_transaction_detail");
        HasKey(rd => new { rd.ReservationId, rd.TransactionLineNumber });

        Property(rd => rd.ReservationId).HasColumnName("reservation_id");
        Property(rd => rd.TransactionLineNumber).HasColumnName("transaction_line_number");

Now for the question/problem

At runtime, I am attempting to get a list of reservations, and each of the related customers on a reservation. Running the following example code:

(this is an IQueryable of ReservationDetail) reservationItems.First().LinkedCustomers.First().CustomerName;

throws an exception on the second First() call because LinkedCustomers has no items. Looking at the DB I am connecting to, there should be one entry in this list of Customers.

Any pointers on where my configuration may be bad?