Unity.MVC register autogenerated dbcontext from ef

Question

I have an autogenerated dbcontext from EF. I've created a partial class of this autogenerated EF class so I can make it implement an interface so that I can pass that interface to my service that will use it. That'll get passed in via Unity.MVC DI framework. When I try to register this interface to the actual class I'm getting an error:

"There is no implicit reference conversion from ContactsEntities to IContactsEntities."

I'm not sure why I'm getting that or how to solve it.

EF Autogen file:

public partial class ContactsEntities : DbContext
    {
        public ContactsEntities()
            : base("name=ContactsEntities")
        {
        }

        protected override void OnModelCreating(DbModelBuilder modelBuilder)
        {
            throw new UnintentionalCodeFirstException();
        }

        public virtual DbSet<Contact> Contacts { get; set; }
    }

My file:

public interface IContactsEntities
{
    DbSet<Contact> Contacts { get; set; }
}

public partial class ContactsEntities : IContactsEntities
{
    public virtual DbSet<Contact> Contacts { get; set; }
}

Unity registering that gives the error:

container.RegisterType<IContactsEntities, ContactsEntities>();