完全不明白

db.users.aggregate(
       [
         {
           $group : {
             _id: { $sum: 1 }
           }
         }
       ]
     )

Error: Printing Stack Trace
    at printStackTrace (src/mongo/shell/utils.js:37:15)
    at DBCollection.aggregate (src/mongo/shell/collection.js:897:9)
    at (shell):1:10
Fri Sep 19 18:51:23.623 aggregate failed: {
    "errmsg" : "exception: invalid operator '$sum'",
    "code" : 15999,
    "ok" : 0