passing `undefined` for the bucket here will break the multi backend I'm pretty sure. It uses it to select which "sub-backend" to pass on the `fix_index` call to: https://github.com/basho/riak_kv/blob/a03f2e96aedf38d462a378d3af4ee541016c9aa5/src/riak_kv_multi_backend.erl#L392