SqlDataSource Optional Parameter Problem

Hi, I'm having some issues with the SqlDataSource. I want to use it
to
populate a GridView, but using an optional parameter to filter the
results.

This is what I have right now (hopefully haven't made any typos
- can't
copy/paste):

<asp:SqlDataSource ID="test1" runat="server"
SelectCommand="SELECT * FROM
SomeTable WHERE (@MyParam IS NULL OR MyColumn =
@MyParam) ORDER BY
SomeColumn" ConnectionString="<%
ConnectionStrings:MyConnString %>" >
<SelectParameters>
 
<asp:ControlParameter Name="MyParam"
ControlID="DropDownList1"
PropertyName="SelectedValue" Type="String"
ConvertEmptyStringToNull="True"
DefaultValue=""
/>
</SelectParameters>
</asp:SqlDataSource>

<asp:DropDownList
ID="DropDownList1" runat="server" AutoPostBack="True">
  <asp:ListItem
Selected="True" Value="">All</asp:ListItem>
 
<asp:ListItem>AnotherValue</asp:ListItem>
 
<asp:ListItem>SomeElse</asp:ListItem>
 
<asp:ListItem>Whatever</asp:ListItem>
</asp:DropDownList>

<asp:GridView
ID="GV" runat="server" DataSourceID="test1"
AutoGenerateColumns="False"
DataKeyNames="SomeID"
  <columns>
    <asp:BoundField
DataField="SomeColumn" HeaderText="A Title"
SortExpression="SomeColumn"
/>
   (more bound columns here...)
 
</columns>
</asp:GridView>

When I test the SQL query in
the query designer it works (returns only rows
having the value passed as a
parameter when one is specified, otherwise it
returns all rows), so it seems
like that part is OK. The "All" (as in
"return all"/no filtering) entry in
the DropDownList has a value of a zero
lenght string, and the
ControlParameter has the convert empty string to null
to true (and the
default value is the same), so it should get converted to a
null when "All"
is selected, hence returning all rows. But it doesn't work.
It works fine for
all the entries with text, but the zero lenght string
somehow doesn't work -
I get no results at all instead of it returning all
rows (but the query
itself worked fine when I tested it).

What am I missing? I just can't
find what I'm doing wrong. Any ideas/hints?
(I also need to do the same with
an ObjectDataSource, so hopefully I can get
this to work!) I can't think of
an easy way to find out if the zero lenght
string gets converted to a null or
not (I've even tried adding OR @MyParam =
'' to the query and it still didn't
work....) Right now I'm stuck....

(Also posted this question on microsoft.public.dotnet.framework.adonet newsgroup)

Thanks a lot in advance for the
replies!

Carl B.