cannot throw error from ES6 generator

error_in_generator.es.js

function getStockPrice() {
   seq.throw('oops! could not get stock price');
   // seq.next(80);
}

function executeTrade() {
   setTimeout(function() {
      console.log("trade done");
      seq.next(true);
   }, 300);
}

var stockTrade = function* () {
   try {
      var price = yield getStockPrice();
      
      console.log('toots');
      if(price > 45) {
         executeTrade();
      } else {
         console.log('trade not done');
      }
   } catch(ex) {
      // always says:
      // caught error: Generator is already running
      // expected "oops! could not get stock price"
      console.log("caught error: " + ex.message);
   }
}

var seq = stockTrade();

seq.next(); // {value: undefined, done: true}

working example at https://gist.github.com/deepak/f0d2066bbedd51c3add3#file-wrap_errors_for_generators-es-js-L10