Method Error 500

Using the CascagingDropDownList extender I keep getting the following message returned from my web service.

The aspx code is as follows:
1 <asp:UpdatePanel ID="UpdatePanel1" runat="server">
2 <ContentTemplate>
3 Development:<br />
4 <asp:DropDownList ID="ddlDev" runat="server" Width="304px">
5 </asp:DropDownList><br />
6 <br />
7 Plot Number:<br />
8 <asp:DropDownList ID="ddlPlotNo" runat="server" Width="216px">
9 </asp:DropDownList> ...