How do I get Entity Framework to handle optional table splitting?

Say I have these classes:

public class temp
{
    [Key, Required]
    public Guid tempid { get; set; }

    [Required(AllowEmptyStrings = true), MaxLength(50)]
    public string name { get; set; }

    [MaxLength(100)]
    public string filename { get; set; }

    [Required]
    public virtual tempContent tempContent { get; set; }

    public temp()
    {
        tempid = Guid.NewGuid();
    }

    public override string ToString()
    {
        return name;
    }

    public override bool Equals(object obj)
    {
        if (obj == null || GetType() != obj.GetType()) return false;

        temp other = (temp)obj;

        return tempid == other.tempid;
    }

    public override int GetHashCode()
    {
        return tempid.GetHashCode();
    }
}

public class tempContent
{
    [Key, Required]
    public Guid tempid { get; set; }

    public byte[] filecontents { get; set; }

    [Required]
    public virtual temp temp { get; set; }
}

Then I have:

    public DbSet<temp> temps { get; set; }
    public DbSet<tempContent> tempContents { get; set; }

I define the split with this:

        modelBuilder.Entity<tempContent>(o => o.ToTable("temps"));

        modelBuilder.Entity<temp>(temp =>
        {
            temp.HasOne(o => o.tempContent).WithOne(o => o.temp).HasForeignKey<temp>(o => o.tempid);
            temp.Navigation(o => o.tempContent).IsRequired();
        });

When I try add an instance without the filename/content, I get :

Microsoft.EntityFrameworkCore.DbUpdateConcurrencyException: The database operation was expected to affect 1 row(s), but actually affected 0 row(s); data may have been modified or deleted since entities were loaded.