I agree. If you start out with an empty graph, the Lucene index is not created until you add the first vertex. It is not easy to find why Lucene would only allow alphanumeric characters for names of indices (implicit assumption in the JanusGraph Preconditions check) and whether this still holds for the currently used version of Lucene.

If you want, you can create an issue for it or even provide a PR.

Best wishes,    Marc