Entity Framework 7 trouble creating migration because navigation property with the same name exists

I'm trying to create my first migration using Entity Framework 7 with code first development and I'm getting the following error:

The property 'Email' cannot be added to the entity type 'UserDTO' because a navigation property with the same name already exists on entity type 'UserDTO'.

My environment is: 1) Visual Studio 2015 2) Entity Framework v7.0.0-rc1-final 3) Code first development 4) Using the fluent API, not data annotations

I can't figure out what the root cause of the issue is but I've got a few ideas. My RoleDTO class is supposed to use its email address property as its PK, and it also has a collection of RoleDTO objects as a property.

Below is the only class I have inheriting from DbContext right now. I've commented out the other entity I have to try and reduce the problem.

class StorageContext : DbContext
{
    public DbSet<UserDTO> Users { get; set; }

    protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
    {
        optionsBuilder.UseSqlServer("MySQLServerConnectionString")
    }

    protected override void OnModelCreating(ModelBuilder modelBuilder)
    {
        modelBuilder.Entity<UserDTO>().HasKey(entity => entity.Email);
        //modelBuilder.Entity<UserDTO>().HasMany(entity => entity.Roles);
    }
}

And here is the UserDTO class. Can anyone see what's causing the error when I'm trying to make the migration?

internal class UserDTO
{
    public EmailAddress Email { get; private set; }

    public string FullName { get; private set; }

    public string UserName { get; private set; }

    public virtual ICollection<string> Roles { get; private set; }

    // more below here like a constructor and some basic methods

If I switch the key for UserDTO to a plain string instead of the complext object EmailAddress it looks like it gets past that error and I get a different, equally interesting one:

The property 'Roles' on entity type 'Microsoft.SRE.NavisionOnline.ConfigurationAutomation.DAL.SQLEntities.UserDTO' has not been added to the model or ignored.