MapReduce with single reduce optimization fails when merging results in sharded cluster

Resmoke test: (function() { const st = new ShardingTest({ shards: 2 , mongos: 1 , other: { mongosOptions: {setParameter: {mrEnableSingleReduceOptimization: true }}, shardOptions: {setParameter: {mrEnableSingleReduceOptimization: true }}, } });   const mongosDB = st.s0.getDB(jsTestName()); const mongosColl = mongosDB[jsTestName()];   assert .commandWorked(mongosDB.dropDatabase()); assert .commandWorked(mongosDB.adminCommand({enableSharding: mongosDB.getName()})); st.ensurePrimaryShard(mongosDB.getName(), st.shard0.shardName);   assert .commandWorked( mongosDB.adminCommand({shardCollection: mongosColl.getFullName(), key: {_id: 1 }}));   // Split the collection into 4 chunks: [MinKey, -100), [-100, 0), [0, 100), [100, MaxKey). assert .commandWorked(mongosDB.adminCommand({split: mongosColl.getFullName(), middle: {_id: 0 }}));   // Move the [0, 100) and [100, MaxKey) chunks to shard1. assert .commandWorked(mongosDB.adminCommand( {moveChunk: mongosColl.getFullName(), find: {_id: 50 }, to: st.shard1.shardName})); assert .commandWorked(mongosDB.adminCommand( {moveChunk: mongosColl.getFullName(), find: {_id: 150 }, to: st.shard1.shardName}));   assert .commandWorked(mongosColl.insert({_id: - 1 })); const map = function() { emit( 0 , {val: "mapped value" }); };   const reduce = function(key, values) { return {val: "reduced value" }; };   let res = assert .commandWorked(mongosDB.runCommand( {mapReduce: mongosColl.getName(), map: map, reduce: reduce, out: {inline: 1 }})); assert .eq(res.results[ 0 ], {_id: 0 , value: {val: "mapped value" }});   assert .commandWorked(mongosColl.insert({_id: 1 }));   res = assert .commandWorked(mongosDB.runCommand( {mapReduce: mongosColl.getName(), map: map, reduce: reduce, out: {inline: 1 }})); assert .eq(res.results[ 0 ], {_id: 0 , value: {val: "reduced value" }});   st.stop(); }());