title: Office.js Word - window.external.RegisterEvent is not a function
body: <p>I am developing a Word 2016 Addin and I have received an error from a client (logged via <a href="https://sentry.io" rel="noreferrer">Sentry</a>) that I'm having trouble reproducing. It occurred in a Dialog window on macOS Sierra 10.12.6 in Word 16.11.</p>
  
  <pre><code>window.external.RegisterEvent is not a function.
  (In 'window.external.RegisterEvent(n,t,i,r)', 'window.external.RegisterEvent' is undefined)
  </code></pre>
  
  <p>The error originated from the word API script <code>https://appsforoffice.microsoft.com/lib/1/hosted/word-mac-16.00.js</code> and looking at the debug code on Github, it appears to be coming from <a href="https://github.com/OfficeDev/office-js/blob/release/dist/word-mac-16.00.debug.js#L4365" rel="noreferrer">this line</a>.</p>
  
  <p><strong>Does anyone have any insight into why this may be occurring?</strong> It hasn't happened on any other platform and I'm having trouble reproducing it locally.</p>
  
  <p>Thanks in advance. I'm happy to provide more information if required.</p>
  
  <p><strong>NOTE</strong>: <em>This error occurred during the Addin submission review process causing a failed submission - hence the need to track down the source.</em></p>