In mapReduce map function emitting a document with 'undefined' or 'null' key in sharded collection fails

t = db.mr_undef;

t.drop();

outname = "mr_undef_out";

out = db[outname];

out.drop();

t.insert({x: 0});

var m = function() {

    emit(undefined, this.x);  // the key can be 'null' also

};

var r = function(k, v) {

    total = 0;

    for (i in v) {

        total += v[i];

    }

    return total;

};

res = t.mapReduce(m, r,  {out: outname});

printjson(res);

printjson(out.find().toArray());

assert.eq(1, out.find({_id: {$type: 10}}).itcount(), "A2");

[js_test:mr_undef] 2017-03-15T03:59:11.699-0400 {       

[js_test:mr_undef] 2017-03-15T03:59:11.699-0400         "result" : "mr_undef_out",

[js_test:mr_undef] 2017-03-15T03:59:11.699-0400         "counts" : {

[js_test:mr_undef] 2017-03-15T03:59:11.700-0400                 "input" : NumberLong(1),

[js_test:mr_undef] 2017-03-15T03:59:11.700-0400                 "emit" : NumberLong(1),

[js_test:mr_undef] 2017-03-15T03:59:11.700-0400                 "reduce" : NumberLong(0),

[js_test:mr_undef] 2017-03-15T03:59:11.700-0400                 "output" : NumberLong(0)

[js_test:mr_undef] 2017-03-15T03:59:11.700-0400         },

[js_test:mr_undef] 2017-03-15T03:59:11.700-0400         "timeMillis" : 51,

[js_test:mr_undef] 2017-03-15T03:59:11.700-0400         "timing" : {

[js_test:mr_undef] 2017-03-15T03:59:11.700-0400                 "shardProcessing" : 24,

[js_test:mr_undef] 2017-03-15T03:59:11.700-0400                 "postProcessing" : 26

[js_test:mr_undef] 2017-03-15T03:59:11.701-0400         },

[js_test:mr_undef] 2017-03-15T03:59:11.701-0400         "shardCounts" : {   

[js_test:mr_undef] 2017-03-15T03:59:11.701-0400                 "huracan:20003" : {

[js_test:mr_undef] 2017-03-15T03:59:11.702-0400                         "input" : 1, 

[js_test:mr_undef] 2017-03-15T03:59:11.702-0400                         "emit" : 1,

[js_test:mr_undef] 2017-03-15T03:59:11.702-0400                         "reduce" : 0,

[js_test:mr_undef] 2017-03-15T03:59:11.702-0400                         "output" : 1     <---------------this suggests there was an output document.

[js_test:mr_undef] 2017-03-15T03:59:11.703-0400                 }

[js_test:mr_undef] 2017-03-15T03:59:11.703-0400         },

[js_test:mr_undef] 2017-03-15T03:59:11.703-0400         "postProcessCounts" : { 

[js_test:mr_undef] 2017-03-15T03:59:11.703-0400                 "huracan:20003" : {

[js_test:mr_undef] 2017-03-15T03:59:11.703-0400                         "input" : NumberLong(0),

[js_test:mr_undef] 2017-03-15T03:59:11.703-0400                         "reduce" : NumberLong(0),

[js_test:mr_undef] 2017-03-15T03:59:11.704-0400                         "output" : NumberLong(0)

[js_test:mr_undef] 2017-03-15T03:59:11.704-0400                 }

[js_test:mr_undef] 2017-03-15T03:59:11.704-0400         },

[js_test:mr_undef] 2017-03-15T03:59:11.704-0400         "ok" : 1

[js_test:mr_undef] 2017-03-15T03:59:11.704-0400 }

[js_test:mr_undef] 2017-03-15T03:59:11.704-0400 [ ]    <------- this is from the "printjson(out.find().toArray());" which shows no output document.

....

[js_test:mr_undef] 2017-03-15T03:59:11.707-0400 assert: [1] != [0] are not equal : A2

[js_test:mr_undef] 2017-03-15T03:59:11.707-0400 doassert@src/mongo/shell/assert.js:18:14

[js_test:mr_undef] 2017-03-15T03:59:11.707-0400 assert.eq@src/mongo/shell/assert.js:54:5

[js_test:mr_undef] 2017-03-15T03:59:11.707-0400 @jstests/core/mr_undef.js:26:1

[js_test:mr_undef] 2017-03-15T03:59:11.707-0400

[js_test:mr_undef] 2017-03-15T03:59:11.707-0400 2017-03-15T03:59:11.707-0400 E QUERY    [thread1] Error: [1] != [0] are not equal : A2 :

[js_test:mr_undef] 2017-03-15T03:59:11.708-0400 doassert@src/mongo/shell/assert.js:18:14

[js_test:mr_undef] 2017-03-15T03:59:11.708-0400 assert.eq@src/mongo/shell/assert.js:54:5

[js_test:mr_undef] 2017-03-15T03:59:11.708-0400 @jstests/core/mr_undef.js:26:1

[js_test:mr_undef] 2017-03-15T03:59:11.708-0400 failed to load: jstests/core/mr_undef.js