after set profilling level, cannot get any results from db.system.profile.find()

> use test
switched to db test
> db.system.profile.find()
> db.setProfilingLevel(2)

{ "was" : 2, "slowms" : 100, "ok" : 1 }

> db.test.insert(

{"a":1}

)
> db.system.profile.find()
>