PRIMARY> db.test.insertOne({foo:1})
{
	"acknowledged" : true,
	"insertedId" : ObjectId("5b6e094002374d6d7383f3e2")
}
PRIMARY> db.test.aggregate([{"$project": {"_id": false, "foo": true}}])
{ "foo" : 1 }
PRIMARY> db.test.aggregate([{"$project": {"_id": false, "foo": true}}, {'$out': 'output-collection'}])
assert: command failed: {
	"operationTime" : Timestamp(1533937989, 2),
	"ok" : 0,
	"errmsg" : "Failed to extract unique key from input document: { foo: 1.0 }",
	"code" : 50905,
	"codeName" : "Location50905",
	"$clusterTime" : {
		"clusterTime" : Timestamp(1533937989, 2),
		"signature" : {
			"hash" : BinData(0,"AAAAAAAAAAAAAAAAAAAAAAAAAAA="),
			"keyId" : NumberLong(0)
		}
	}
} : aggregate failed

$ ~/Downloads/mongodb-macos-x86_64-enterprise-4.1.1-293-g3927b18/bin/mongod --version
db version v4.1.1-293-g3927b18
git version: 3927b187d07cd5e9c9ebba26ecf9500b261f57fc
allocator: system
modules: enterprise
build environment:
    distarch: x86_64
    target_arch: x86_64