var a = new Timestamp();
db.test.insert(a);
mongo shell can run this insert command it doesn't need validate _id?

var b = new String("123");
db.test.insert(b)
this can also done
but if we run find() , it return error
res_malie:PRIMARY> db.test.find();
{ "_id" : ObjectId("57be5382bd9465a0ef04aaed"), "a" : 5, "b" : 5 }
{ "_id" : ObjectId("57be53b4bd9465a0ef04aaee"), "a" : 2 }
{ "_id" : ObjectId("57be5382bd9465a0ef04aaee") }
{ "_id" : ObjectId("5874e3dea1369cbc0d880fbf"), "timestamp" : Timestamp(1484055375, 0) }
{ "_id" : ObjectId("5874ead321ce10a26f6b7d9b"), "a" : "a" }
{ "_id" : ObjectId("5874ead621ce10a26f6b7d9c"), "a" : "A" }
Timestamp(0, 0)
Timestamp(0, 0)
Timestamp(0, 0)
Timestamp(0, 0)
2017-01-11T14:20:31.084+0800 E QUERY    [thread1] SyntaxError: missing ] after element list :
tojsonObject@src/mongo/shell/types.js:708:13
tojson@src/mongo/shell/types.js:653:21
DBQuery.prototype.shellPrint@src/mongo/shell/query.js:533:63
shellPrintHelper@src/mongo/shell/utils.js:490:1
@(shell2):1:1

Error: SyntaxError: missing ] after element list :
tojsonObject@src/mongo/shell/types.js:708:13
tojson@src/mongo/shell/types.js:653:21
DBQuery.prototype.shellPrint@src/mongo/shell/query.js:533:63
shellPrintHelper@src/mongo/shell/utils.js:490:1
@(shell2):1:1



I found in mongorestore and other tools have a --objcheck default option

but mongod I find nothing
so
it's a mongo shell bug ?