Strange LINQ to SQL .Union() error

(See at the bottom for a full repro)

With the following entity ...

[Table]
internal sealed class Employee
{
    private EntityRef<Employee> manager;

    [Column(IsPrimaryKey = true, IsDbGenerated = true)]
    private int Id;

    [Column]
    private int? ManagerId;

    [Column]
    internal bool IsOverpaid;

    [Association(Name = "Manager_Subordinate", Storage = "manager", ThisKey = "ManagerId", IsForeignKey = true)]
    internal Employee Manager
    {
        get { return this.manager.Entity; }
        set { this.manager.Entity = value; }
    }
}

... this query fails with a NotSupportedException, with the message "Types in Union or Concat are constructed incompatibly.":

var overpaidTopManagers =
    from employee in context.Employees
    where employee.IsOverpaid && (employee.Manager == null)
    select employee;
var managersWithOverpaidSubordinates =
    from employee in context.Employees
    where employee.IsOverpaid && (employee.Manager != null)
    select employee.Manager;
var query = overpaidTopManagers.Union(managersWithOverpaidSubordinates);

I don't really understand why, both queries produce the same type of entity, so it shouldn't be a problem to union them?

Full repro follows:

using System;
using System.Data.Linq;
using System.Data.Linq.Mapping;
using System.Linq;

internal static class Program
{
    private static void Main(string[] args)
    {
        using (var context = new Context("Whatever.sdf"))
        {
            if (!context.DatabaseExists())
            {
                context.CreateDatabase();
            }

            var overpaidTopManagers =
                from employee in context.Employees
                where employee.IsOverpaid && (employee.Manager == null)
                select employee;
            var managersWithOverpaidSubordinates =
                from employee in context.Employees
                where employee.IsOverpaid && (employee.Manager != null)
                select employee.Manager;
            var query = overpaidTopManagers.Union(managersWithOverpaidSubordinates);

            // This throws a NotSupportedException with the Message
            // "Types in Union or Concat are constructed incompatibly."
            foreach (var manager in query)
            {
                Console.WriteLine(manager.ToString());
            }
        }
    }
}

[Table]
internal sealed class Employee
{
    private EntityRef<Employee> manager;

    [Column(IsPrimaryKey = true, IsDbGenerated = true)]
    private int Id;

    [Column]
    private int? ManagerId;

    [Column]
    internal bool IsOverpaid;

    [Association(Name = "Manager_Subordinate", Storage = "manager", ThisKey = "ManagerId", IsForeignKey = true)]
    internal Employee Manager
    {
        get { return this.manager.Entity; }
        set { this.manager.Entity = value; }
    }
}

internal sealed class Context : DataContext
{
    internal Table<Employee> Employees;

    internal Context(string fileOrServerOrConnection) : base(fileOrServerOrConnection)
    {
        this.Employees = this.GetTable<Employee>();
    }
}