Solution 1:

I had the same problem and I found that I had not called

 base.OnModelCreating(modelBuilder);

in my DbModel

Solution 2:

The following method, OnModelCreating, will create a working context. Not sure if it is the mapping you want though.

protected override void OnModelCreating(DbModelBuilder modelBuilder)
{
    base.OnModelCreating(modelBuilder);

    modelBuilder.Entity<Content>()
        .HasMany(c => c.Editors)
        .WithOptional()
        .WillCascadeOnDelete(false);

    modelBuilder.Entity<Content>()
        .HasRequired(c => c.Owner)
        .WithOptional()
        .WillCascadeOnDelete(false);

    modelBuilder.Entity<IdentityUserLogin>().HasKey<string>(l => l.UserId);
    modelBuilder.Entity<IdentityRole>().HasKey<string>(r => r.Id);
    modelBuilder.Entity<IdentityUserRole>().HasKey(r => new { r.RoleId, r.UserId });
}