Why do I have a circular reference when I call my webservice?

Hi, I have a webservice that calls returns data from a database.  It calls into the DAL and returns data.

I have verified the query works and can call it from my codebehind asp.net c# page.  It works fine.

If I invoke it from the standard webservice page, it works fine and click the button to run my webservice, it works fine and returns an XML page.

However, If I call it with the below code I get:

Sys.Net.WebServiceFailedException:  The server method GetMyData failed with the following error: System.InvalidOperationException -- A circular reference was detected while serializing an object of type 'DataSet1+DataTable1DataTable'. 

Here is the code.  Thank you for any help:

in my page,1 function OnLookup()
2 {
3 var stb = document.getElementById("_symbolTextB ...