C# - Sorting Gridview Bound To Linq SP

I have a grid bound to a linqed SP thus:

Session["results"] = db.spGetCaseByNumberOrSurname(txtCaseNum.Text.Trim(), null).ToList();
gvResults.DataSource = Session["results"];

on the sorting of it, i would like to be able to do this..

protected void gvResults_Sorting(object sender, GridViewSortEventArgs e)
{
string sortExpression = e.SortExpression;
List<spGetCaseByNumberOrSurnameResult> data = Session["results"] as List<spGetCaseByNumberOrSurnameResult>;
if (sd == SortDirection.Ascending)
{
sd = SortDirection.Descending;
gvResults.DataSource = data.OrderBy(d => d.GetType().GetProperty(sortExpression));
}
else
{
sd = SortDirection.Ascending;
gvResults.DataSource = data.OrderByDescending(d => d.GetType().GetProperty(sortExpression));
}
gvResults.DataBind();
}

sadly that doesnt do any sorting at all..it in fact errors with "The data source does not support server-side data paging."