I'm migrating a MongoDB monitoring tool from mgo to new official mongo driver.

One of my MongoDB version uses the version 2.6 (yes I know, it's very old, and my team is migrating it)

In my code, I'm doing a "find one" query on the "$cmd.sys.inprog" collection.

With the old mgo library, it was working fine. With the official mongo driver, it returns an error message:

query failure: {"$err": "bad numberToReturn (0) for $cmd type ns - can only be 1 or -1","code": {"$numberInt":"16978"}}

I provided in attachment my old and new code.

Any idea ?