sourcemaps sometimes not working

Hi,

I had an exception yesterday (https://app.raygun.io/dashboard/1cnqy6/errors/248484535) with this stacktrace:

Unable to get property 'getAttribute' of undefined or null reference at ? line 37, column 457 (https://dev-www.mojitopf.com/app/:37)

For some reason Raygun didn't give me the usual sourcemapped stack trace. Would be great if Raygun could explain why the sourcemap is unavailable - is my application misconfigured somehow? Raygun couldn't download the sourcemap? The sourcemap was invalid? Or maybe it never works with exceptions from IE11?

Thanks, Darren.