Issues by Submitter is broken

(Bug, Closed -> Fixed, Priority: Critical, Test Status: No automated tests yet , Reported By Justin du Coeur, )
Summary: Not sure what's wrong, but I'm getting an internal error when I try it.
The problem seems to be in the expression:
Issue._instances -> _groupBy(Reported By)
Something about trying to group by Reported By is failing, even though grouping by other fields works. Something about using Person in that position?