MongoDB平均返回空值

数据库的一部分如下:

 { "_id" : 1, "car" : { "engineSize" : 1.5 }, "addresses" : [ "X", "A" ] } 

 { "_id" : 6, "car" : { "engineSize" : 1.5 }, "addresses" : [ "X" ] }

 { "_id" : 2, "car" : { "engineSize" : 1.3 } }

 { "_id" : 5, "car" : { "engineSize" : 1.4 } } 

 { "_id" : 3, "car" : { "engineSize" : 1 } }

我用来检查平均值的命令是:

db.project.aggregate([{$group : {_id: null, Average: {$avg: "$engineSize"}}}])

 { "_id" : null, "Average" : null }

关于如何让它返回正确的值(即1.24)有什么建议吗?