GridView Sorting with LING

Given the following HTML and codebehind:

 <form id="form1" runat="server">
    <div>
        <asp:GridView ID="Gridview1" runat="server" AutoGenerateColumns="False" BackColor="White"
            BorderColor="#E7E7FF" BorderStyle="None" BorderWidth="1px" CellPadding="3" GridLines="Horizontal"
            AllowSorting="true">
            <RowStyle BackColor="#E7E7FF" ForeColor="#4A3C8C" />
            <Columns>
                <asp:BoundField DataField="Ename" HeaderText="Employee Name" SortExpression="Ename" />
                <asp:BoundField DataField="AnnSal" HeaderText="Annual Salary" DataFormatString="{0:c}"
                    SortExpression="AnnSal" />
            </Columns>
            <FooterStyle BackColor="#B5C7DE" ForeColor="#4A3C8C" />
            <PagerStyle BackColor="#E7E7FF" ForeColor="#4A3C8C" HorizontalAlign="Right" />
            <SelectedRowStyle BackColor="#738A9C" Font-Bold="True" ForeColor="#F7F7F7" />
            <HeaderStyle BackColor="#4A3C8C" Font-Bold="True" ForeColor="#F7F7F7" />
            <AlternatingRowStyle BackColor="#F7F7F7" />
        </asp:GridView>
    </div>
    </form>

   Protected Sub form1_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles form1.Load

        If Not IsPostBack Then

            Dim query = From p In GetEmployeeList() Select p.Ename, AnnSal = (p.Sal * 12)

            Me.Gridview1.DataSource = query

            Me.Gridview1.DataBind()

        End If

    End Sub

The grid is populated OK but clicking on a field to sort generate this error:

The GridView 'Gridview1' fired event Sorting which wasn't handled.

How can event sorting handler be coded here?

Thanks for any help.