Expand into a DTO's navigation property using AutoMapper (using EF)

I am trying to expose a simple data model using an WebAPI OData service. My data is stored in a database which I am accessing using EF. Because the tables in my DB use different property names and there are too many fields which are needed in my DTO's, I use AutoMapper to map (or rather, ProjectTo) from DB objects to DTO's.

DTO's are as follows:

public class OrderDTO
{
    [Key]
    public int SalesOrderNumber { get; set; }
    //Navigation
    public virtual IEnumerable<OrderLineDTO> OrderLines { get; set; }
}

public class OrderLineDTO
{
    [Key]
    [ForeignKey("Order")]
    public int SalesOrderNumber { get; set; }
    [Key]
    public int LineNumber { get; set; }
    //Navigation
    public virtual OrderDTO Order { get; set; }
}

When I access a specific order I get my repsonse as expected (ie: the requested order):

http://localhost/.../Orders(salesOrderNumber=1)

When trying to expand the orderLines property using following request I get the below response:

http://localhost/.../Orders(salesOrderNumber=1)?$expand=orderLines

{
  "@odata.context": "http://localhost/.../$metadata#Orders/$entity",
  "division": "STAND",
  "salesOrderNumber": 1,
  "orderLines@odata.context": "http://localhost/.../$metadata#Orders(salesOrderNumber=1)/orderLines",
  "orderLines": []
}

I suspect the reason my orderLines object is empty is because AutoMapper doesn't take the expand into consideration when converting from DB objects to DTO's. My entity query looks like this:

_dbContext.ORDERS
  .Include("LINES")
  .Where(o => (o.ORD_NUM == salesOrderNumber))
  .ProjectTo<OrderDTO>()
  .FirstOrDefault();

AutoMapper has mappings for both OrderDTO and OrderLineDTO. Configured as follows:

CreateMap<ORDERS, OrderDTO>()
  .ForMember(dest => dest.SalesOrderNumber, opt => opt.MapFrom(src => src.ORD_NUM));

CreateMap<LINES, OrderLineDTO>()
  .ForMember(dest => dest.SalesOrderNumber, opt => opt.MapFrom(src => src.ORD_NUM))
  .ForMember(dest => dest.LineNumber, opt => opt.MapFrom(src => src.LIJNNR));

It seems to me that this should be enough to be able to $expand into my orderLines but I can't get this to work. What am I missing here?