This is basically the inverse issue of#683 and I think it could use some official documentation.

If you migrate from 7.x to 9.x with an existing GCM app, you will get this error during startup (at least when using Redis, it seems to come from a call toRpush::Daemon::Store::Redis.all_apps). The last comment in the linked issue highlights you should go from 7 -> 8 first. Then I presume manually delete the GCM app. Then move from 8 -> 9.

I didnt see anything in the readme or changelog about this, but hit it when we rolled out to our staging environment. For our use case we dont keep notifications around long term, so we didnt actually have any GCM notifications to clean up when deleting the exisiting app, but others might so perhaps some documentation about how to cleanly delete an existing app in addition to the 7.x -> 8.x notes (or is it safe to just callapp.destroy?).