Custom Search without a "WHERE" clause fails when used as a smart group

I created a custom search that does not have a "Where" clause. (This is intentional, as it has a complex join and the criteria is part of the join clause)

The custom search works fine if I run it directly, but it fails when used as a smart group.

From debugging, it seems the code in file "CRM/Contact/BAO?GroupContactCache.php" assumes there is a where clause in the SQL, as th SQL fragment it adds start with "AND ..."