Problem

Currently tag names aren't validated in BlockBuilder#tag or BlockBuilder#tagBlockAndItem and possibly in other places. While this itself is not a huge issue, this allows faulty behavior to happen when undefined (or a similarly stringified strange value) is passed in.

Requested Solution

Either a regex check or some other string validation method that raises an error in the logs, besides a generic stack trace.

Hmm, I wonder whether our changes to ResourceLocation type wrappers since then might have already fixed this on the side :P