Often not getting stack traces, just "raven in wrapped" errors

I’m using raven.js in my JS browser app and getting lots of the following error. This is all I can see. I’ve grepped by .js’s / my webpack artifacts for “data.indexOf” and “X8je” but found nothing. This is the most common error in my Sentry. How am I supposed to figure out what this is and where it’s coming from?