sharepoint 2010 rest api javascript error: 5009 sys is undefined

I am running into something that I haven't really seen before, and I have been using this approach quite a bit lately for a variety of pages. SharePoint 2010 javascript using REST API to grab data:

getContent("http://myServer/sites/mySite/_vti_bin/ListData.svc/Announcements", thisCallback)

function getContent(whatURL, callback)
{
     var request = new Sys.Net.WebRequest();
     request.set_httpVerb("GET");
     request.set_url(whatURL);
     request.get_headers()["Accept"] = "application/json";
     request.add_completed(callback);
     request.invoke();
}

and parse json and process with callback. but now i am getting console error "Sys is undefined" and I am using the same approach within the same site on different pages and it works? any ideas? just for checking i would add a simple callback and it still doesn't work

function thisCallback(response, eventArgs)
{
      // wish i could get some results to work with
}

appreciated as always.