AutoCompleteExtender - Cannot deserialize. The data does not correspond to a valid JSON.

I am using the AutoCompleteExtender in the latest AjaxControlToolKit from Nuget in a ASP.NET 4.5 web form project.

It is a very basic demo app just to show the control.  Everything works fine, but running in Visual Studio 2012 the Debug output keeps sayingJavascript runtime error: Sys.ArgumentException: Cannot deserialize.  The data does not correspond to valid JSON.everytime the web method is called.

As I said, the auto complete is working fine and names are showing.  A Fiddler trace would show something like{"prefixText":"ma","count":20} on the POST data and a result like:

{"d":["kingkong@gmail.com","ronnie@email.com",maggie@whitehall.gov.uk]}

At the moment this exception is just an irritation but is it a sign of something more serious?

Thanks.