Request data not being added to callback

The 'mapping' of function parameters is working, but not the addition of the request data.

Using the example module:

function js_callback_examples_js_callback_get_uid($first_name) {
  print $first_name; // Works
}

Opposed to:

function js_callback_examples_js_callback_get_uid($not_a_request_param) {
  print $not_a_request_param; // NULL
}

This is also true in the multi-variable scenario, I've added a second parameter in the form:

function js_callback_examples_js_callback_get_uid($first_name, $not_a_request_param) {
  print $first_name; // works
  print $not_a_request_param; // NULL
}