OData can't expand

I'm trying to use OData and I'm having a problem with using the $expand function.

I have 3 entities: Event, Order, Ticket

They are setup like this: Event:

    public int Id { get; set; }
    public string UserId { get; set; }
    public string Name { get; set; }
    public string ShopName { get; set; }
    public string ImageUrl { get; set; }
    public string Description { get; set; }
    public DateTimeOffset Start { get; set; }
    public DateTimeOffset End { get; set; }
    public virtual ICollection<Order> Orders { get; set; } = new Collection<Order>();

Order:

    public int Id { get; set; }
    [ForeignKey("Event")]
    public int EventId { get; set; }

    public string CustomerFirstName { get; set; }
    public string CustomerLastName { get; set; }
    public string CustomerEmail { get; set; }
    public string CustomerPhoneNumber { get; set; }
    public string CustomerAddress { get; set; }
    public string CustomerHouseNumber { get; set; }

    public string TransactionId { get; set; }
    public TransactionStatus Status { get; set; }

    public DateTimeOffset OrderedOn { get; set; }

    public virtual Event Event { get; set; }
    public virtual ICollection<Ticket> Tickets { get; set; } = new Collection<Ticket>();

Ticket:

    public int Id { get; set; }

    [ForeignKey("Order")]
    public int OrderId { get; set; }

    public string Barcode { get; set; }

    public virtual Order Order { get; set; }

If I want to retrieve an Event and expand Orders and then also expand Tickets, there's no issue. As long as I use Event as a starting point.

However, if I select an Order (which is working fine) and want to expand Tickets, I get the following error: "'DbQuery`1' cannot be serialized using the ODataMediaTypeFormatter."

Again, this ONLY happens if I want to retrieve the Tickets for a specific Order. If I select the Event and expand from there, everything is working fine.

Does anyone know what I'm doing wrong? I can't seem to figure it out.