using the beginning ajax with asp.net book

I followed chapter 7 try it out on page 162. The image is clicked, the cs method is invoked, the path to the image is returned but the ChangeMe_Callback jscript functions response has and error. The response.error.description equals 'undefined'. What could be the problem?

Thanks
Glenn

I found the problem. I was reference the AjaxPro.dll library. I'm using ASP.NET 2005 so I needed to reference the AjaxPro.2.dll library. Works as advertised.