asp：DropDownList错误：'DropDownList1'有一个SelectedValue，它是无效的，因为它在项目列表中不存在

I have a asp.net 2.0 web site with numerous asp:DropDownList controls. The DropDownList control contains the standard info city, state, county etc... info. In addition to the standard codes the site also has custom codes that the users can configure themselves. For example a animal dropdown may contain the values Dog, Cat, Fish, ect...I have a asp.net 2.0 web site with numerous asp