how to set value in dropdownlist

hi,
I tried to set default value to null, but the problem is when I do DDReportFormat.SelectedValue,
it is actually the string value "null", not really a null value. 
 
<asp:DropDownList ID="DDReportFormat" runat="server">
<asp:ListItem Text="Un specified" Value=null ></asp:ListItem>
<asp:ListItem>PDF</asp:ListItem>
<asp:ListItem>EXCEL</asp:ListItem>
</asp:DropDownList>
 
thanks
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