How to Pass null value to database from DropDownList

I tried like this its not working
<asp:DropDownList ID="dllcardtype" runat="server" Width="190px"> <asp:ListItem Value="">Select Type</asp:ListItem> </asp:DropDownList> <asp:DropDownList ID="dllcardtype" runat="server" Width="190px"> <asp:ListItem Value="Null">Select Type</asp:ListItem> </asp:DropDownList>