Unspecified Error in IE8/9

I am trying to log errors in IE8 & IE9 but when an error happens I get an Unspecified error in the console and the error is not logged. It works fine in Chrome and Firefox.

I’ve created a quick page to demo the error:

<script src="https://cdn.ravenjs.com/3.17.0/raven.js" crossorigin="anonymous"></script>
<script type="text/javascript">
<button onclick="a=b">Click me</button>

Errors from the IE9 console:

SCRIPT5009: 'b' is undefined
File: test.html, Line: 9, Column: 18
SCRIPT16389: Unspecified error.
File: raven.js, Line: 2342, Column: 13

Is there anything I need to do to get sentry to work with IE8/9?