AjaxControlToolkit.CascadingDropDown missing required Category property value for cddMonth.
Parameter name: Category

Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code.

Exception Details: System.ArgumentException: AjaxControlToolkit.CascadingDropDown missing required Category property value for cddMonth.
Parameter name: Category
i don't have anything to pass as a category.

[System.Web.Script.Services.ScriptMethod]

monthList.Add(new CascadingDropDownNameValue("February", "2"));

monthList.Add(new CascadingDropDownNameValue("April", "4"));

monthList.Add(new CascadingDropDownNameValue("June", "6"));

monthList.Add(new CascadingDropDownNameValue("August", "8"));

monthList.Add(new CascadingDropDownNameValue("October", "10"));

monthList.Add(new CascadingDropDownNameValue("December", "12"));

Taken from this site: http://www.asp.net/AJAX/AjaxControlToolkit/Samples/Walkthrough/CCDWithDB.aspx 

At runtime, the extender will make callbacks to a web service we specify (which is decorated with the System.Web.Script.Services.ScriptService attribute). In that web service, it expects a WebMethod with the following signature (note that parameter names must match too!):
[WebMethod]
public CascadingDropDownNameValue[] GetColorsForModel(
string knownCategoryValues,
string category)
The knownCategoryValues parameter will return a string containing the currently selected category values, as well as the category to retrieve values for. For example, if the extender is populating the "Color" field, you will be passed the values for the "Make" and "Model" fields, as well as "Color" to specify the field to return values for.

The CascadingDropDown class has a helper method for unpacking the category values: StringDictionary kv = CascadingDropDown
.ParseKnownCategoryValuesString(knownCategoryValues);

This method will return a StringDictionary containing the name/value pairs of the currently selected values. So imagine you've got a database with tables for the Make (manufacturer), Model, and Color information, and you're accessing that database through a DataSet to which you've added methods for getting each set of values.

mac,

Just put something, "text" would do, as the Category property value in your Cascading drop down extender.  This is just to make your call match the signature of your web service.  You don't have to actually use the value you are passing in your implementation.

James