SharePoint 2013 WebProxy return 403 - Calls to WebProxy without app context not allowed

Greetings:

I'm trying to use the web proxy feature in SP 2013. Here is the code:

var hostWebUrl = "http://contoso.com/somepage.html";
var request;
var response;

function successHandler() {
  if (response.get_statusCode() == 200) { //everything is good
    $("#DivTagId").html(response.get_body());
  } else {
    var msg = "Response returned: " + response.get_statusCode();
    msg += "<br/>" + response.get_body();
    $("#DivTagId").html(msg);
  }
}
function errorHandler() {
  var msg = "Error!<br/>" + response.get_body();
  $("#DivTagId").html(msg);
}

function loadData () {
  try {
    var context = SP.ClientContext.get_current();
    request = new SP.WebRequestInfo();
    request.set_url(hostWebUrl);
    request.set_method("GET");
    request.set_headers({"Accept":"text/html"});
    response = SP.WebProxy.invoke(context, request);
    context.executeQueryAsync(successHandler, errorHandler);
  } catch (e) { 
    console.log(e);
  }
}

SP.SOD.executeFunc("sp.js","SP.ClientContext",loadData);

The code executes and returns with a status code of 403 and a request body that says: "Calls to WebProxy without an app context are not allowed." I've used SP.ClientContext.get_current (see above). What am I missing?

- BigCountrix