Ickle query return unexpected (many) results for an optional numeric field in inner object if checked for "is null"

If an object is declared like this:
https://github.com/wfink/infinispan.playground.ickle/blob/master/HotRodClient/src/main/resources/playground/company.proto

and the query is the following (in the linked project)
runIckleQuery(qf, "from playground.Company c where c.employee.age is null");
the result return with Company object which does not have any Employee.