Subdoc atomic/concurrency broken

#!/usr/bin/env python

from threading import Thread

from couchbase.bucket import Bucket

import couchbase.subdocument as SD

DOCID = 'subdoc_doc_id'

CONNSTR = 'couchbase://localhost:12000'

ITERATIONS = 200

THREADS = 20

main_bucket = Bucket(CONNSTR)

main_bucket.upsert(DOCID, {'recs':[]})

thrs = []

class Runner(Thread):

    def run(self, *args, **kw):

        cb = Bucket(CONNSTR)

        for x in range(ITERATIONS):

            cb.mutate_in(DOCID, SD.array_append('recs', 1))

thrs = [Runner() for x in range(THREADS)]

[t.start() for t in thrs]

[t.join() for t in thrs]

obj = main_bucket.get(DOCID)

print 'Object has {} items'.format(len(obj.value['recs']))