invalid association mapping

Hi , wondering if anyone has any ideas to this within my class mapping. I think the answer maybe in my db structure.
Anyway, all thoughts very much appreciated.

Invalid association mapping for member 'Asset.DesignImages'. 'DesignImage' is not an entity

I've notice VS studio is giving me warnings:

Warning    1    Value for property "Discriminator Property" cannot be empty.        0    0
Warning    2    Value for property "Inheritance Default" cannot be empty.        0    0

My DataBase
Asset
DesignText (foreignkey to asset.id )
DesignImage (foreignkey to asset.id )

Maybe i'm gettng nulls???

var query =

from d in db.Designs

from a in db.Assets

select new ProductViewHelper()

{

    theDesign = new Design(){DesignId = d.Id, Title = d.Title, DateField = DateTime.Now, PreviewImage = d.PreviewImage},

    theAsset = new Asset(){AssetId = a.Id, DesignId = a.DesignId, Date = a.Date},

    DesignTexts = (from x in a.DesignTexts select new DesignText()

    { DesignTextId = x.Id, Text = x.Text}).ToList(),

    DesignImages = (from y in a.DesignImages select new DesignImage()

    {Id = y.Id}).ToList()

};

        foreach (var p in query)
        {

                Response.Write(p.theAsset.id);


        }

Re: invalid association mapping

This is because the DesignImage table does not have a primary key. Entities are unique objects that need a descriminating key. So, either remove the relationship or add a primary key to the table.