This happens on index.js line 91 - apparently somehow flags is undefined when variation is called in the callback for the ready event. Our error tracker reports that this has happened in just a small fraction of page loads, so it's working most of the time. We're using the ldclient-js npm module v1.1.3.
I think this is another symptom of the same problem as #27: when a request to the LaunchDarkly service returns an error, the LD script gets no flags back, and that causes problems. (This might not even be an issue with LD - it could be that the internet flickered while the request was being made.)