Error: dropdownlist1 has a SelectedValue which is invalid because it does not exist in the list of items. Parameter name: value

HI All,
I am having difficult time to solve the 'dropdownlist has a selectedvalue which is invalid...". I know this is due to a null value in table doesn't exist in the dropdownlist box listitem. So I tried to add code "<asp:ListItem style="display:none" Value=""></asp:ListItem>" and AppendDataBoundItems="True"  and removed SelectedValue='<%# Bind("drpPlan1") %>' 
But then I don't know how to save the form changes back to the database in formview edititemtemplate.
My code as below:
<asp:DropDownList ID="drpPlan1" runat="server" DataSourceID="SqlDataSource1" AppendDataBoundItems="true"
                                DataTextField="Program" DataValueField="Code" 
               ...