TypeError：xhrReq.abort不是函数

Question

var xhrReq;
function tpvAjaxPost(json) {
  if(xhrReq !== undefined) {
      xhrReq.abort();
      xhrReq = undefined;
  }
  xhrReq = $.post("inc/ajax/tpv.php", json, function (data) {
      if(typeof (data['message']) !== 'undefined') {
          tpvHandleErrors(data['message']);
      } else {
          tpvHandleResults(json['action'], data);
      }
  }, "json").always(function() { xhrReq = undefined; });
}

为什么我得到：TypeError：xhrReq.abort不是函数????