DateReader exception when using delegates in LINQ Where clause

Question

I am using a delegate to construct the where clause of a linq query. Here is an example:

public Func<Order, bool> WhereFunc(string search)
{
   return o => o.OrderNumber.Contains(search) || o.Part.Name.Contains(search);
} 

I then call this function like so:

entities = _orderRepo.Query
                        .Where(WhereFunc(search))
                        .OrderBy(OrderByFunc(orderBy))
                        .Skip(start*pageSize)
                        .Take(pageSize)
                        .ToList()
                        .Select(OrderDisplay.New);

I get the following error being given from executing the WhereFunc:

{"There is already an open DataReader associated with this Command which must be closed first."}

Could anyone help me understand why this is happening and what I can do to mitigate it? I assume it's something to do with accessing the proxy entities in the delegate function but I'm not exactly sure...

UPDATE

If I change WhereFunc to the following:

public Func<Order, bool> WhereFunc(string search)
{
   return o => o.OrderNumber.Contains(search);
} 

Then then exception is not thrown - hence my assumption it's something to do with proxy entities.

UPDATE

Here are my entities:

public class Order : EntityBase<int>
{
    public string OrderNumber { get; set; }
    public virtual Part Part { get; set; }

    [ForeignKey("Part")]
    public int PartId { get; set; }

}

public class Part : EntityBase<int>
{
    public string Name { get; set; }
    public virtual ICollection<Order> Orders { get; set; }
}