overzealous empty path detection?

[pfps]

The query https://qlever.cs.uni-freiburg.de/wikidata/1glIyB

SELECT DISTINCT * WHERE { 
<http://www.wikidata.org/entity/Q11629> 
( (<http://www.wikidata.org/prop/direct/P279> / ((<http://www.wikidata.org/prop/direct/P279>)*|(<http://www.wikidata.org/prop/direct/P31>)*))
| (<http://www.wikidata.org/prop/direct/P31> / ((<http://www.wikidata.org/prop/direct/P279>)*|(<http://www.wikidata.org/prop/direct/P31>)*)) )
?x1 }

causes

This query might have to evaluate the empty path, which is currently not supported. In file "/local/data-ssd/qlever/qlever-code/src/engine/TransitivePathImpl.h " at line 160

But the equivalent https://qlever.cs.uni-freiburg.de/wikidata/5D68x8

SELECT DISTINCT * WHERE { 
<http://www.wikidata.org/entity/Q11629> 
( (<http://www.wikidata.org/prop/direct/P279> / <http://www.wikidata.org/prop/direct/P279>*)
| (<http://www.wikidata.org/prop/direct/P279> / <http://www.wikidata.org/prop/direct/P31>*)
| (<http://www.wikidata.org/prop/direct/P31> / <http://www.wikidata.org/prop/direct/P279>*)
| (<http://www.wikidata.org/prop/direct/P31> / <http://www.wikidata.org/prop/direct/P31>*) )
?x1 }

works fine.

Why is the former a problem?

There quite a few queries like this in the paths section of the wdbench benchmark.