Entity Framework 4.1 - Override Entity (DBSet) with Filter

I'm trying to do something which should be relatively easy, but i just dont know how to construct it.

I have a Generated Entity which I'd like to override by adding a Linq Where statement.

Herewith the partial for the Context :

public partial class MyEntities: DbContext
{
    public MyEntities()
        : base("name=MyEntities")
    {
    }    
    public DbSet<Assignee> Assignees { get; set; }
}

I've created a new partial of MyEntities and tried the following

public override DbSet<Assignee> Assignees 
{
    get
    {
        return this.Assignees.Where(z => z.IsActive == true);
    }
    set; 
}

but this throws an ambiguity error (which is obvious).

How can I accomplish this?

Thanks