AddToManager problem

   I've retrieved entities from my database via my DevForce Silverlight data model.  The actual database table has a PK column and it is defined as a tinyint.

The retrieved entities are bound to a datagrid in which I allow new rows to be added.  My event handler is as follows:

private static void C1DataGrid_BeginningNewRow(object sender, DataGridBeginningNewRowEventargs e)
{
    try
    {
        BaseEntity baseEntity = e.Item as BaseEntity;
        baseEntity.EntityAspect.AddToManager();
    }

    catch
    {
        throw;
    }
}

The AddToManager call is failing with the following error "Value was either too large or too small for an unsigned byte".

We have other tables that have a PK column as an int that work fine.  What happens is when I say add a new row in the datagrid I am returned a number such as -100.  Eventually when I commit the entities back to the database the PK column value is resolved.

How do I handle this in the case where the PK on the table is a datatype such as a tinyint?