title: Dayjs custom locale throws Formatting exceptions
body: <p>I want to create a <strong>custom locale</strong> and use it with <strong>DayJs</strong>. and the procedure is mentioned here in <a href="https://github.com/iamkun/dayjs/blob/master/docs/en/I18n.md#customize" rel="nofollow noreferrer">docs</a> </p>
  
  <p>But, when I follow the procedure and create my own locale, I am <strong>unable to format the date after.</strong></p>
  
  <p>Here is the <strong>JSFiddle</strong> to inspect the same.
  <a href="https://jsfiddle.net/5o4pwbtc/" rel="nofollow noreferrer">https://jsfiddle.net/5o4pwbtc/</a></p>
  
  <p>Here is the <strong>GitHub</strong> Issue for the same: <a href="https://github.com/iamkun/dayjs/issues/746" rel="nofollow noreferrer">https://github.com/iamkun/dayjs/issues/746</a></p>
  
  <p><div class="snippet" data-lang="js" data-hide="false" data-console="true" data-babel="false">
  <div class="snippet-code">
  <pre class="snippet-code-js lang-js prettyprint-override"><code>/*  like mentioned here
  https://github.com/iamkun/dayjs/blob/dev/docs/en/I18n.md#installation
  */
  const locale = {
    formats: {
      // abbreviated format options allowing localization
      LTS: 'h:mm:ss A',
      LT: 'h:mm A',
      L: 'MM/DD/YYYY',
      LL: 'MMMM D, YYYY',
      LLL: 'MMMM D, YYYY h:mm A',
      LLLL: 'dddd, MMMM D, YYYY h:mm A',
      // lowercase/short, optional formats for localization
      l: 'D/M/YYYY',
      ll: 'D MMM, YYYY',
      lll: 'D MMM, YYYY h:mm A',
      llll: 'ddd, MMM D, YYYY h:mm A'
    },
    relativeTime: {
      name: 'en',
      future: '%s',
      past: '%s',
      s: 'now',
      m: 'a minute ago',
      mm: '%d minutes ago',
      h: 'an hour',
      hh: '%d hours', // e.g. 2 hours, %d been replaced with 2
      d: 'a day',
      dd: '%d days',
      M: 'a month',
      MM: '%d months',
      y: 'a year',
      yy: '%d years'
    }
  }
  
  dayjs.locale(locale);
  
  dayjs(1575872723701).format() 
  // Uncaught TypeError: Cannot read property '10' of undefined</code></pre>
  <pre class="snippet-code-html lang-html prettyprint-override"><code>&lt;script src="https://unpkg.com/dayjs@1.8.17/dayjs.min.js"&gt;&lt;/script&gt;</code></pre>
  </div>
  </div>
  </p>