DataSet does not support System.Nullable<>

I also used following code from googling:

var result = from r in ObjContext.Users
select new
{
ID=r.ID,
Username= r.Username==null?"":r.Username,
Password= r.Password==null?"":r.Password,
SecLevel = r.SecLevel == null ? 3 : r.SecLevel,

Firstname = r.Firstname == null ? "" : r.Firstname,
Lastname = r.Lastname == null ? "" : r.Lastname,
BCC = r.BCC == null ? "" : r.BCC,
SenderEmail = r.SenderEmail == null ? "" : r.SenderEmail,

};
dt = result.CopyToDataTable();

but same porblem has comes.