sqldatasource object reference not set to an instance of an object

i am having an unusual problem
i have a sqldatasource in my code:
<asp:SqlDataSource ID="sdsEventGuests" Runat="server" ProviderName="System.Data.SqlClient" DataSourceMode="DataSet" SelectCommand="spGetGuests" SelectCommandType="StoredProcedure"> <SelectParameters> <asp:Parameter Name="Mode" Type="String" DefaultValue="" /> <asp:Parameter Name="GFWhere" DefaultValue="" Type="String" /> </SelectParameters> </asp:SqlDataSource>
in the code behind:
Me.sdsEventGuests.SelectParameters.Clear() Me.sdsEventGuests.SelectParameters("Mode").DefaultValue = "GuestFinder"  Me.sdsEventGuests.SelectParameters("GFWhere").DefaultValue = strWhere
when i run the page, i get:
object reference not set to an instance of an objec ...