How to use `explain("executionStats")` in aggregate?

I base on Meteor1.7 + Mongo 3.6
I would like to use explain("executionStats") in aggregate function in Mongo IDE (Booster), but don’t work


// --- Get error
	"message" : "explain failed: MongoError: Each element of the 'pipeline' array must be an object",
	"stack" : "script:6:6",
	"code" : 14


db.testing.aggregate(pipeline, { explain: true })

I work fine, but don’t have executionStats property info.