TypeError: Cannot read property 'priority' of undefined

[yujiosaka]

What is the current behavior?

In rare possibility (in my case less than 3%), the crawler fails enqueueing next request and never stops.
The error goes like this:

TypeError: Cannot read property 'priority' of undefined
    at lowerBound (/Users/yujiosaka/work/headless-chrome-crawler/cache/session.js:71:63)
    at lowerBound (/Users/yujiosaka/work/headless-chrome-crawler/lib/helper.js:117:11)
    at SessionCache.enqueue (/Users/yujiosaka/work/headless-chrome-crawler/cache/session.js:71:15)
    at PriorityQueue.push (/Users/yujiosaka/work/headless-chrome-crawler/lib/priority-queue.js:34:17)
    at PriorityQueue.<anonymous> (/Users/yujiosaka/work/headless-chrome-crawler/lib/helper.js:178:23)
    at HCCrawler._push (/Users/yujiosaka/work/headless-chrome-crawler/lib/hccrawler.js:271:17)
    at _skipRequest.then.skip (/Users/yujiosaka/work/headless-chrome-crawler/lib/hccrawler.js:545:16)
    at process._tickCallback (internal/process/next_tick.js:103:7)

If the current behavior is a bug, please provide the steps to reproduce

The following scripts causes errors sometimes.

const HCCrawler = require('headless-chrome-crawler');

HCCrawler.launch({
  maxDepth: 3,
  maxConcurrency: 10,
  allowedDomains: ['www.emin.co.jp'],
  evaluatePage: (() => window.document.title),
  onSuccess: (result => { // 成功時に評価される関数
    console.log(`${result.options.url}\t${result.result}`);
  }),
})
  .then(crawler => {
    crawler.queue('https://www.emin.co.jp/');
    crawler.onIdle()
      .then(() => crawler.close());
  });

What is the expected behavior?

• Do not cause an error
• Execute all queued requests
• Successfully close the crawler

What is the motivation / use case for changing the behavior?

Please tell us about your environment:

• Version: 1.3.4
• Platform / OS version: MacOS
• Node.js version: v6.4.0