JSON Error: "A circular reference was detected while serializing an object of type ..."

I'm trying to do pretty much exactly what this example (in the link shown below) shows. And I get the error of "A circular reference was detected...".

http://ajax.asp.net/docs/tutorials/ConsumingWebServicesWithAJAXTutorial.aspx - Passing and Returning Complex Types - 2nd example

Does anyone know why this error occurs?  Looking at the example (from the link), I would think that would generate the error, but it doesn't.

Essentially what I'm trying to do is:

In Javascript:



Collect form values from the HTML via Javascript

build an object of a specific type

set the properties

send the javascript object to a webservice method as an argument

then return the result from the webservice as a JSON object
Her ...