Write command with invalid write concern crashes mongos in a mixed cluster.

>>> c.test.command('update', 'test', updates=[SON([('q', {'_id': {'$in': [2, 99997]}}), ('u', {'$set': {'foo': 'bar'}}), ('upsert', False), ('multi', True)])])

{u'nModified': 1, u'ok': 1, u'n': 2}

>>> 

>>> c.test.command('update', 'test', updates=[SON([('q', {'_id': {'$in': [3, 99996]}}), ('u', {'$set': {'foo': 'bar'}}), ('upsert', False), ('multi', True)])], writeConcern={'w': 2, 'wtimeout': 100})

Traceback (most recent call last):

  File "<stdin>", line 1, in <module>

  File "pymongo/database.py", line 435, in command

    uuid_subtype, compile_re, **kwargs)[0]

  File "pymongo/database.py", line 335, in _command

    for doc in cursor:

  File "pymongo/cursor.py", line 1017, in next

    if len(self.__data) or self._refresh():

  File "pymongo/cursor.py", line 961, in _refresh

    self.__uuid_subtype))

  File "pymongo/cursor.py", line 885, in __send_message

    res = client._send_message_with_response(message, **kwargs)

  File "pymongo/mongo_client.py", line 1201, in _send_message_with_response

    raise AutoReconnect(str(e))

pymongo.errors.AutoReconnect: connection closed

2014-03-12T13:54:56.212-0700 [mongosMain] MongoS version 2.6.0-rc2-pre- starting: pid=92669 port=27017 64-bit host=devtop.local (--help for usage)

2014-03-12T13:54:56.212-0700 [mongosMain] db version v2.6.0-rc2-pre-

2014-03-12T13:54:56.213-0700 [mongosMain] git version: 27afe048247d1ca3d32858bd9047fc92e97efea8

...

2014-03-12T13:55:50.499-0700 [Balancer] distributed lock 'balancer/devtop.local:27017:1394657696:16807' unlocked. 

2014-03-12T13:55:52.067-0700 [conn1] ERROR: Uncaught std::exception: vector::_M_range_check, terminating

2014-03-12T13:55:52.067-0700 [conn1] dbexit:  rc:100

mongos> sh.status()

--- Sharding Status --- 

  sharding version: {

	"_id" : 1,

	"version" : 4,

	"minCompatibleVersion" : 4,

	"currentVersion" : 5,

	"clusterId" : ObjectId("531fe9146e3028d5a974d0b6")

}

  shards:

	{  "_id" : "shard0000",  "host" : "localhost:37017" }

	{  "_id" : "shard0001",  "host" : "localhost:37018" }

  databases:

	{  "_id" : "admin",  "partitioned" : false,  "primary" : "config" }

	{  "_id" : "test",  "partitioned" : true,  "primary" : "shard0000" }

		test.test

			shard key: { "_id" : 1 }

			chunks:

				shard0001	3

				shard0000	4

			{ "_id" : { "$minKey" : 1 } } -->> { "_id" : 0 } on : shard0001 Timestamp(2, 0) 

			{ "_id" : 0 } -->> { "_id" : 15093 } on : shard0001 Timestamp(3, 0) 

			{ "_id" : 15093 } -->> { "_id" : 31137 } on : shard0001 Timestamp(4, 0) 

			{ "_id" : 31137 } -->> { "_id" : 47181 } on : shard0000 Timestamp(4, 1) 

			{ "_id" : 47181 } -->> { "_id" : 69233 } on : shard0000 Timestamp(3, 2) 

			{ "_id" : 69233 } -->> { "_id" : 95999 } on : shard0000 Timestamp(3, 4) 

			{ "_id" : 95999 } -->> { "_id" : { "$maxKey" : 1 } } on : shard0000 Timestamp(3, 5)