Hi,

I just reply the similar problem on http://forums.asp.net/p/1805523/4988723.aspx/1?p=True&t=634731045133627387,

Pleas focus the following code at he that reply, you need to define the value as @Html.HiddenFor to make it not null  

@Html.HiddenFor(model => model.PersonID)
        @Html.HiddenFor(model => model.Discriminator)

Have fun