My jQuery ajax call is failing with an undefined error. My js code looks like this:

$.ajax({
   type: "POST",
   url: "Data/RealTime.ashx",
   data: "{}",
   contentType: "application/json; charset=utf-8",
   dataType: "json",
   timeout: 15000,
   dataFilter: function(data, type) {
       alert("RAW DATA: " + data + ", TYPE: "+ type);
       return data;
   },
   error: function(xhr, textStatus, errorThrown) {
       alert("FAIL: " + xhr + " " + textStatus + " " + errorThrown);
   },
   success: function(data) {
       alert("SUCCESS");
   }
});

My ajax source is a generic ASP.NET handler:

[WebService(Namespace = "http://my.website.com")]
[WebServiceBinding(ConformsTo = WsiProfiles.BasicProfile1_1)]
public class RealTime : IHttpHandler
{
    public void ProcessRequest(HttpContext context)
    {
        context.Response.ContentType = "application/json";
        context.Response.Write("{ data: [1,2,3] }");
        context.Response.End();
    }

    public bool IsReusable
    { get { return false; } }
}

Now, if I return an empty object ("{ }") in my handler, the call will succeed. But when I return any other JSON object, the call fails.

The dataFilter handler shows that I am receiving a correct object. Firebug shows the response as expected, and the JSON tab shows that the object is parsed correctly.

So what could be the cause?