mongos tries to connect to the secondary to create the index if `rs.slaveOk()` is set.

Start a sharded cluster on 4.2: mongos> db.version() 4.2.0 Connect to the mongos and run rs.slaveOk() . mongos> rs.slaveOk() After that, the insert succeeds - meaning that the mongos is connecting to the primary to insert the document. mongos> db.docs.insert({x:1}) WriteResult({ "nInserted" : 1 }) But ensureIndex fails - the mongos is trying to connect to a secondary to run ensureIndex . mongos> db.docs.ensureIndex({x:1}) { "raw" : { "shard01/localhost:20002,localhost:20003,localhost:20004" : { "ok" : 0, "errmsg" : "not master", "code" : 10107, "codeName" : "NotMaster" } }, "code" : 10107, "codeName" : "NotMaster", "ok" : 0, "errmsg" : "not master", "operationTime" : Timestamp(1568357018, 1), "$clusterTime" : { "clusterTime" : Timestamp(1568357018, 1), "signature" : { "hash" : BinData(0,"AAAAAAAAAAAAAAAAAAAAAAAAAAA="), "keyId" : NumberLong(0) } } }