Schema specified is not valid. Errors: The relationship 'EntityA_EntityBs' was not loaded because the type 'EntityB' is not available

Background

I'm working on a program that allows a third-party program to query a database using Entity Framework. There are several tables in the database, but no foreign keys. I used Entity Framework Fluent API to map the pertinent tables to entities.

Entities

namespace App.Entities
{
    public class Ticket
    {
        public int Id { get; set; }

        public virtual SalesOrder SalesOrder { get; set; }
        public int SalesOrderId { get; set; }
    }

    public class SalesOrder
    {
        public int Id { get; set; }

        public virtual ICollection<Ticket> Tickets { get; set; }
        public virtual ICollection<Ticket> Lines { get; set; }
    }
}

Mappings

The column and table name mappings are missing since, in my opinion, they are not important.

namespace App.Mappings
{
    public class TicketMap : EntityTypeConfiguration<Ticket>
    {
        public TicketMap() {}
    }

    public class SalesOrderMap : EntityTypeConfiguration<SalesOrder>
    {
        public SalesOrderMap()
        {
            HasMany(t => t.Tickets)
                .WithRequired(t => t.SalesOrder)
                .HasForeignKey(t => t.SalesOrderId);

            HasMany(t => t.Lines)
                .WithRequired(t => t.SalesOrder)
                .HasForeignKey(t => t.SalesOrderId);
        }
    }
}

Entity Framework has appropriately registered the entities and their mappings.

Problem

The following is what I'm gettingMetadataException when I attempt to do a search:

Schema specified is not valid. Errors: The relationship 'App.SalesOrder_Tickets' was not loaded because the type 'App.Ticket' is not available.

Keep in mind that the namespace ofTicket the message above is incorrect. Actually, the entity class is inApp.Entities.Ticket . I'm not sure, though, if this is connected to the issue or not.

Attempts I've Made

1. Verifying that the relationship looks to be set up properly
2. checking the model for duplicate classes as recommended in this statement
3. Verifying the entities' alignment with the namespace recommended in this response