$_POST is empty after XMLHttpRequest

Question

I get a response from the page I'm trying to reach however $_POST is always empty. The GET equivalent works and so does posting using jQuery's AJAX. What the heck is going on here?

function search_now() {
  var payload = JSON.stringify({ q: input_box.value });
  var xhttp = new XMLHttpRequest();
  xhttp.onreadystatechange = function() {
    if (xhttp.readyState == 4) {
      results_display.innerHTML = xhttp.responseText;
    }
  };
  xhttp.open('POST', '/search-results/', true);
  xhttp.setRequestHeader("Content-type", "application/json");
  // xhr.setRequestHeader('Content-type', 'application/x-www-form-urlencoded');
  xhttp.send(payload);
}