SQLDataSource With Query Strings

Hi, I have a SQLDataSource binding to a GridView and can come to the page either with or without a query string attached:

 /ProjectManagement/reporting/project.aspx

/ProjectManagement/reporting/project.aspx?portfolio=3

When it comes with a query string, I can see in SQL Server profiler it executes and I get all the right data. When it is an empty string, or with no "?portfolio=" on it, it won't even execute against SQL server. Any ideas?

<asp:GridView ID="grid" runat="server" Width="600px"

ShowHeader="false"

AutoGenerateColumns="false"

DataSourceID="DSportfolio"

AllowSorting = "true"

AllowPaging = "true">

<Columns>

<asp:TemplateField>

<ItemTemplate>

 .............

</ItemTemplate>

</asp:TemplateField>

</Columns>

</asp:GridView>

<asp:SqlDataSource ID="DSportfolio" ConnectionString="<%$ AppSettings:SQLConnection1 %>"

SelectCommand="uspSELECT_PROJECT"

SelectCommandType="StoredProcedure"

runat="server">

<SelectParameters>

<asp:QueryStringParameter Name="p_PORTFOLIOID" QueryStringField="portfolio" />

</SelectParameters>

</asp:SqlDataSource>

Thanks,

James