find_and_modify returns OperationError instead of DuplicateKeyError

import pymongo

c = pymongo.Connection()

db = c['test']

db['test'].drop()

db['test'].ensure_index('a', unique=True)

db['test'].insert({'a': '1', 'b': 1}, safe=True)

try:

    db['test'].insert({'a': '1', 'b': 2}, safe=True)

except pymongo.errors.DuplicateKeyError:

    print 'insert gives pymongo.errors.DuplicateKeyError'

try:

    db['test'].find_and_modify(query={'a': '1', 'b': 2}, update={'$set': {'c': 1}}, upsert=True)

except pymongo.errors.DuplicateKeyError:

    print 'find_and_modify gives pymongo.errors.DuplicateKeyError'

except pymongo.errors.OperationFailure:

    print 'find_and_modify gives pymongo.errors.OperationFailure'

insert gives pymongo.errors.DuplicateKeyError

find_and_modify gives pymongo.errors.DuplicateKeyError

insert gives pymongo.errors.DuplicateKeyError

find_and_modify gives pymongo.errors.OperationFailure