SQL DataSource doesn't show any record

this is my SP:

ALTER proc [dbo].[autherlist]
@l nvarchar(1)=''
as 
IF @l = ''
BEGIN
SELECT [author_id], [F-name], [L-name]FROM [authors] ORDER BY [L-name]
end
else
begin
SELECT [author_id], [F-name], [L-name]FROM [authors] where [L-letter] = @l ORDER BY [L-name]
end

and this my datasource:

<asp:SqlDataSource ID="ds" runat="server" 
    ConnectionString="<%$ ConnectionStrings:ariaquoteConnectionString %>" 
    SelectCommand="autherlist" SelectCommandType="StoredProcedure">
    <SelectParameters>
        <asp:RouteParameter DefaultValue="" Name="l" RouteKey="letter" Type="String" />
    </SelectParameters>
</asp:SqlDataSource>

problem is this: SP execute well in sql manager, and with ado.net but with datasource show no record!
where is the problem?

Tags: C#, SQL, ADO.NET, ASP.NET