I need to access to matchInstance collection and get players' id in a match. but my script return no value at all. am i missing something?

I tried those:

var players = Spark.find("matchInstance", {"_id": matchID}, {"players":1 , "_id" : 0});

var players = Spark.find("SYSTEM.matchInstance", {"_id": matchID}, {"players":1 , "_id" : 0});