Note that I haven't been able to reproduce this locally, only on CI. Here is the patch that I ran. The code I'm using is as follows:  client = Mongo::Client. new (uri_string, database: "pets" ) client[ 'cats' ].delete_many({}) client[ 'dogs' ].delete_many({}) client[ 'cats' ].insert_one({ name: "Whiskers" , color: "white" , age:10, adoptable: true }) client[ 'dogs' ].insert_one({ name: "Pebbles" , color: "Brown" , age:10, adoptable: true }) client.close   adoptablePetsCount = 0 # Start Snapshot Query Example 1 # note that I create the client twice because this part needs to be in the example client = Mongo::Client. new (uri_string, database: "pets" ) client.start_session(snapshot: true ) do |session| adoptablePetsCount = client[ 'cats' ].aggregate([ { "$match" : { "adoptable" : true } }, { "$count" : "adoptableCatsCount" } ], session: session).first[ "adoptableCatsCount" ] adoptablePetsCount += client[ 'dogs' ].aggregate([ { "$match" : \{ "adoptable" : true } }, { "$count" : "adoptableDogsCount" } ], session: session).first[ "adoptableDogsCount" ] puts adoptablePetsCount end