Error when aggregating with Mongo

Hi All- I’m trying to do some aggregating with Mongo and I keep getting the error "Exception while invoking method ‘pieChart’ MongoError: exception $group does not support inclusion-style expressions"
I have installed meteorhacks:aggregate package and have aggregations working in other areas of my app. This is server side code, btw.
I have googled to no avail, and I can’t find SO posts with an answer regarding this type of error. Any help would be greatly appreciated. Thanks!

 let twentyFourHourArray = [];
        let time = moment('12:00 am');
        let pipeline = [
          {
            $match: { 
              $and: [
                {'data.hour': time},
                {'data.campaign_id': { $in: campaignIds } }
              ]
            }
          },
          {$group: {
            _id: time,
            spend: {$sum: "$data.spend"},
            clicks: {$sum: "$data.clicks"},
            impressions: {$sum: "$data.impressions"},
            likes: {$sum: "$data.like"}
            }
          }
        ];
        for (var i = 0; i <= 24; i++) {
          let result = HourlyBreakdowns.aggregate(pipeline);
          result[0]['cpc'] = result[0].spend / result[0].clicks;
          result[0]['cpm'] = result[0].spend / (result[0].impressions / 1000);
          result[0]['cpl'] = result[0].spend / result[0].likes;
          twentyFourHourArray.push(result);
          time = time.add(1, 'hour');
        }

Complete Error:

I20160328-16:10:11.694(-4)? Exception while invoking method ‘pieChart’ MongoError: exception: $group does not support inclusion-style expressions
I20160328-16:10:11.695(-4)? at Object.Future.wait (/Users/prwelber/.meteor/packages/meteor-tool/.1.1.13-beta.12.nzn13w++os.osx.x86_64+web.browser+web.cordova/mt-os.osx.x86_64/dev_bundle/server-lib/node_modules/fibers/future.js:420:15)
I20160328-16:10:11.695(-4)? at packages/meteor/helpers.js:119:1
I20160328-16:10:11.695(-4)? at [object Object].Mongo.Collection.aggregate (packages/meteorhacks_aggregate/packages/meteorhacks_aggregate.js:19:1)
I20160328-16:10:11.695(-4)? at [object Object].pieChart (server/methods/charts.js:170:37)
I20160328-16:10:11.695(-4)? at maybeAuditArgumentChecks (packages/ddp-server/livedata_server.js:1701:12)
I20160328-16:10:11.695(-4)? at packages/ddp-server/livedata_server.js:711:19
I20160328-16:10:11.696(-4)? at [object Object]..extend.withValue (packages/meteor/dynamics_nodejs.js:56:1)
I20160328-16:10:11.696(-4)? at packages/ddp-server/livedata_server.js:709:40
I20160328-16:10:11.696(-4)? at [object Object].
.extend.withValue (packages/meteor/dynamics_nodejs.js:56:1)
I20160328-16:10:11.696(-4)? at packages/ddp-server/livedata_server.js:707:46
I20160328-16:10:11.697(-4)? - - - - -
I20160328-16:10:11.697(-4)? at Object.toError (/Users/prwelber/.meteor/packages/npm-mongo/.1.4.40-beta.12.l4l530++os+web.browser+web.cordova/npm/node_modules/mongodb/lib/mongodb/utils.js:114:11)
I20160328-16:10:11.697(-4)? at /Users/prwelber/.meteor/packages/npm-mongo/.1.4.40-beta.12.l4l530++os+web.browser+web.cordova/npm/node_modules/mongodb/lib/mongodb/db.js:1196:31
I20160328-16:10:11.697(-4)? at /Users/prwelber/.meteor/packages/npm-mongo/.1.4.40-beta.12.l4l530++os+web.browser+web.cordova/npm/node_modules/mongodb/lib/mongodb/db.js:1905:9
I20160328-16:10:11.698(-4)? at Server.Base._callHandler (/Users/prwelber/.meteor/packages/npm-mongo/.1.4.40-beta.12.l4l530++os+web.browser+web.cordova/npm/node_modules/mongodb/lib/mongodb/connection/base.js:453:41)
I20160328-16:10:11.698(-4)? at /Users/prwelber/.meteor/packages/npm-mongo/.1.4.40-beta.12.l4l530++os+web.browser+web.cordova/npm/node_modules/mongodb/lib/mongodb/connection/server.js:488:18
I20160328-16:10:11.698(-4)? at [object Object].MongoReply.parseBody (/Users/prwelber/.meteor/packages/npm-mongo/.1.4.40-beta.12.l4l530++os+web.browser+web.cordova/npm/node_modules/mongodb/lib/mongodb/responses/mongo_reply.js:68:5)
I20160328-16:10:11.698(-4)? at [object Object]. (/Users/prwelber/.meteor/packages/npm-mongo/.1.4.40-beta.12.l4l530++os+web.browser+web.cordova/npm/node_modules/mongodb/lib/mongodb/connection/server.js:446:20)
I20160328-16:10:11.698(-4)? at [object Object].emit (events.js:95:17)
I20160328-16:10:11.699(-4)? at [object Object]. (/Users/prwelber/.meteor/packages/npm-mongo/.1.4.40-beta.12.l4l530++os+web.browser+web.cordova/npm/node_modules/mongodb/lib/mongodb/connection/connection_pool.js:207:13)
I20160328-16:10:11.699(-4)? at [object Object].emit (events.js:98:17)