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 an application that uses Entity Framework to query a database from a third-party application. The database has a large number of tables and no foreign keys. I've mapped the relevant tables to entities using Entity Framework Fluent API.

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

Note that the column and table name mappings are ommitted since I don't think they're relevant.

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);
        }
    }
}

The entities and their mappings are correctly registered with Entity Framework.

Problem

I'm receiving the following MetadataException when I try to run a query:

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

Note that the namespace of Ticket shown in the above message is wrong. The entity class is actually in App.Entities.Ticket. However, I don't know if this is related to the problem or not.

What I've Tried

1. Checking that the relationship appears to be correctly configured
2. Checking for duplicate classes in the model as suggested in this comment
3. Checking that the entities are in the same namespace as suggested in this answer