I have just add the existing code(Default.aspx,AutoComplete.asmx and its AutoComplete.vb) from this example:
http://www.asp.net/learn/AJAX-videos/video-122.aspx

When I try to run this example, it is perfectly to inserting many number and something random number will be returned. Whether I try to modify the code in my project, the results are all 'undefined'.I have set a break point and the results do not have any problem. Also, when I insert some non-numeric string, it is still working.