DropDownList filter...but with an ALL option

Hi guys,

I trying to filter a gridview with a dropdownlist, which is working fine. However, I want the option to return to viewing all. So, I've changed the DDL as so to add an All option with an empty string as the value:
<asp:DropDownList ID="DropDownList1" runat="server" DataSourceID="detachedSQL" appendDataBoundItems="true"
DataTextField="prevride_location" DataValueField="prevride_location">
<asp:ListItem text="All" Value=""></asp:ListItem>
</asp:DropDownList> However, whenever I select that item the GridView returns no results. Here is my code behind: Imports System.Data.SqlClient

Partial Class ridelist2
Inherits System.Web.UI.Page
Private Sub PopulateGrid(ByVal strLocation As String)
Dim strSQL As String
  'if statement to prevent apostrophes in database entri ...