QueryResult.count not work if jpql have order by

I have a entity which have a field scannedDate, create a repository with method findByXXX() which return a QueryResult.

If I call QueryResult.orderAsc("scannedDate").count(), this error will happen.
ERROR: column "t0.scanneddate" must appear in the GROUP BY clause or be used in an aggregate function

I use openjpa 2.2, We cannot simply remove the "order by", Because if we order by a attribute of associated entity like order by e.foo.bar, JPA engine will generate a inner join, if we simply remove the "order by" the count result will not correct.

Maybe this is not a bug, just a limitation.