N1QL "except" does not work properly when the second set is empty.

I have some buckets with indexes, and some without.

I wrote a query to list buckets that have no indexes:

(select id keyspace_id from system:keyspaces) except (select indexes.keyspace_id from system:indexes);

This query worked correctly when some buckets have indexes. When I ran the query on a new machine with no indexes, however, it returns an empty set. I can replicate that incorrect result with the following query:

(select id keyspace_id from system:keyspaces) except (select indexes.keyspace_id from system:indexes where 1=0);

This returns empty results. It is subtracting an empty set from a non-empty set, so it should return the non-empty set.