Entity Framework throws "Collection was modified" when using observable collection

I have a class Employee, and class Company, which contains a (observable) list of employees. The observable list is derived from List<>, and basically looks like this:

public class ObservableList<T> : List<T>
{
   // declaration of delegates and events here
   new public virtual void Add(T item)
        {
            CollectionChangingEventArgs<T> e = new CollectionChangingEventArgs<T>(new T[] { item }, ObservableListChangeAction.Adding);
            OnAdding(e);
            if (e.CancelAction)
                return;

            base.Add(item);

            CollectionChangedEventArgs<T> ed = new CollectionChangedEventArgs<T>(new T[] { item }, ObservableListChangeAction.Added);
            OnAdded(ed);
        }
}

The events are wired in Company constructor. So the whole thing looks approximately like this:

public class Employee
    {
        // stuff here
    }

    public class Company
    {
        // stuff
        ObservableList<Employee> employees = new ObservableList<Employee>();
        public ObservableList<Employee> Employees { get { return this.employees; } }

        public Company()
        {
            Employees.Adding += new ObservableList<Employee>.CollectionChangingEventHandler(Employees_Adding);
        }

        void Employees_Adding(object sender, CollectionChangingEventArgs<Employee> e)
        {
            // do some checks.. none alter the collection
            *edit*
           // I have a line that does this:
           **employee.Employer = this;**
          //So if the employer was saved prior to this point, it errors out.
        }   
    }

Now, if I do this:

Company c = new Company();
Employee e = new Employee();
c.Employees.Adding(e);
c.Save();

it works fine.

If, however, I save employee first, like this:

   Company c = new Company();
    Employee e = new Employee();
    e.Save();
    c.Employees.Adding(开发者_如何学Pythone);
    c.Save();

then EF throws an exception: Collection was modified; enumeration operation may not execute.

Save() method comes from a base class, basically does this:

  this.dbContext.Set<T>().Add(instance);
  dbContext.SaveChanges();

This is a stack trace:

at System.Collections.Generic.List1.Enumerator.MoveNextRare() at System.Data.Objects.ObjectStateManager.PerformAdd(IList1 entries)

at System.Data.Objects.ObjectStateManager.DetectChanges() at System.Data.Entity.Internal.InternalContext.DetectChanges(Boolean force) at System.Data.Entity.Internal.Linq.InternalSet1.ActOnSet(Action action, EntityState newState, Object entity, String methodName) at System.Data.Entity.Internal.Linq.InternalSet1.Add(Object entity)

at System.Data.Entity.DbSet`1.Add(TEntity entity)

If I change ObservableList to List<> - it works. If I don't hook up events - it works. So basically EF doesn't like the events for some reason. When I'm saving the Company, they're not fired, btw.

I am confused.. Any idea what could be causing this ?