Upgrade from 4.0 to 4.1. Problem with EF 2.1 mapping.

I upgrade plugin from 4.0 to 4.1 and have problem with EF mapping.
I have class in 4.0 what extend base Picture class:

    public partial class PictureExt : Picture
    {
        /// <summary>
        /// Gets or sets the PictureId
        /// </summary>
        public virtual string ExternalUrl { get; set; }
    }
and in 4.0 I done mapping:
        public PictureMap()
        {
            this.ToTable("Picture");
            this.HasKey(p => p.Id);
            this.Property(p => p.PictureBinary).IsMaxLength();
            this.Property(p => p.MimeType).IsRequired().HasMaxLength(40);
            this.Property(p => p.SeoFilename).HasMaxLength(300);
            this.Property(p => p.ExternalUrl).IsOptional();
        }
and all work great

but in 4.1 I done mapping:
        public override void Configure(EntityTypeBuilder<PictureExt> builder)
        {
            builder.ToTable("Picture");
            builder.HasKey(picture => picture.Id);

            builder.Property(picture => picture.MimeType).HasMaxLength(40).IsRequired();
            builder.Property(picture => picture.SeoFilename).HasMaxLength(300);
            builder.Property(p => p.ExternalUrl);
            base.Configure(builder);
        }
and I have exception:
A key cannot be configured on 'PictureExt' because it is a derived type. The key must be configured on the root type 'Picture'. If you did not intend for 'Picture' to be included in the model, ensure that it is not included in a DbSet property on your context, referenced in a configuration call to ModelBuilder, or referenced from a navigation property on a type that is included in the model

I understand what I do wrang, but I dont know how do it right.
Maby something lake it https://stackify.com/new-in-net-core-2-1/#post-19576-_kaymrlea07yf ?
But how implement it?