The type checker has proved that this relation is always empty, independent of the actual contents of the database.

The hardest bugs to diagnose when writing queries occur when the query does not return any results - where do you start debugging? For that reason, the .QL type checker tries very hard to prove at compile time that a query will return an empty result, independent of the contents of the database. When such a proof of emptiness is found, it tries to pinpoint the likely reason through the application of a number of heuristics.

This error message is given only when no likely reason can be found, or the reason that is found is rather too complex for inclusion in a simple error message. Clearly it is undesirable for this message to occur, however, so please submit a bug report whenever you encounter it, and we will try to distill better heuristics from those examples.