group failed : Command failed with error 139: 'Failure to enumerate object'

db.coll.insert({_id: 1, user: {id: 1016, name: "jack" }}) db.coll.insert({_id: 2, user: {id: 1017, name: "rose" }})   db.getCollection( "coll" ).group({     key:{ 'user.id' :1},   reduce: function (doc,value){     value.user = doc.user;      value.order_count++;      return  value; }, initial:{ order_count:0,user:0 }, finalize: function (res){ } })