The declared type of navigation property xxx is not compatible with the result of the specified navigation

I am in the process of porting an 'old' EF project to EF 6.1 Code First. The project uses TPH (table per hierarchie) to store customers. A Customer is a subclass of User and a Order has a customer. A customer belongs to a specific subshop. Like this in UML:

I have mapped the customer with a discriminator column like this:

   modelBuilder.Entity<User>()
            .Map<Customer>(m => m.Requires("IsNormalCustomer").HasValue(1))
            .Map<Printer>(m => m.Requires("IsNormalCustomer").HasValue(2));

So all Customers have a IsNormalCustomer value of '1'. A Printer is also a user but not a customer.

In my order repository I have same code to find all orders of a specific subshop.

orders = GetPendingOrders().Where(o => o.Customer.SubShop.SubShopId == subshopId);

If I don't filter on subshop the query runs fine but with filtering I get the following exception:

System.Data.Entity.Core.EntityCommandCompilationException: An error occurred while preparing the command definition. See the inner exception for details. ---> System.Data.Entity.Core.MetadataException: The declared type of navigation property Fotogoed.Data.Order.Customer is not compatible with the result of the specified navigation. at System.Data.Entity.Core.Query.PlanCompiler.PreProcessor.ValidateNavPropertyOp(PropertyOp op)

Could anyone shed some light on what could be the cause of this error? Feel free to ask more information if needed.