Was testing for something & found this issue, So tried to execute what's given in documentation https://docs.mongodb.com/manual/reference/method/db.collection.updateMany/index.html#examples So when executing aggregation pipeline in updateMany() getting this error every time on v4.2, Tried this on robo3T & mongo shell result is same.  Query : (Same as given in documentation)   Insert :   db.members.insertMany([ { "_id" : 1, "member" : "abc123" , "status" : "A" , "points" : 2, "misc1" : "note to self: confirm status" , "misc2" : "Need to activate" , "lastUpdate" : ISODate( "2019-01-01T00:00:00Z" ) }, { "_id" : 2, "member" : "xyz123" , "status" : "A" , "points" : 60, "misc1" : "reminder: ping me at 100pts" , "misc2" : "Some random comment" , "lastUpdate" : ISODate( "2019-01-01T00:00:00Z" ) } ])   Update : db.members.updateMany( { }, [ { $set: { status: "Modified" , comments: [ "$misc1" , "$misc2" ], lastUpdate: "$$NOW" } }, { $unset: [ "misc1" , "misc2" ] } ] ) Working code (works with update) :     db.members.update({}, [{ $set: { status: "Modified" , comments: [ "$misc1" , "$misc2" ], lastUpdate: "$$NOW" } }, { $unset: [ "misc1" , "misc2" ] }], false , true )