Ever since v6.1.0, you have not been able to use subscriptions as an object, as specified in the documentation.

Doing so produces this warning:

[Vue warn]: The "data" option should be a function that returns a per-instance value in component definitions.

A warning would be one thing, but the subscriptions don't actually work (they never get built). Converting it to a function fixes it. While I personally use it as an object a lot, the fact that a minor upgrade breaks every app I use vue-rx on is the most frustrating part of this bug.