Mongo shell prompt errors when running transactions after overriding "db"

> session = db.getMongo().startSession()

> db = session.getDatabase('test')

test

> session.startTransaction()

> db.foo.insert({a:1})

Given transaction number 0 does not match any in-progress transactions.

WriteCommandError({

	"errorLabels": [

		"TransientTransactionError"

	],

	"operationTime": Timestamp(1529690056, 1),

	"ok": 0,

	"errmsg": "Given transaction number 0 does not match any in-progress transactions.",

	"code": 251,

	"codeName": "NoSuchTransaction",

	"$clusterTime": {

		"clusterTime": Timestamp(1529690056, 1),

		"signature": {

			"hash": BinData(0,"AAAAAAAAAAAAAAAAAAAAAAAAAAA="),

			"keyId": NumberLong(0)

		}

	}

})