TypeError: undefined is not a function with MongoDB

Doing this line:

pseudoIsTaken &= db.users.find({'name': name}).limit(1).size() == 0;

raises an error on y MongoDB server:

        throw message;
              ^
TypeError: undefined is not a function
    at C:\Users\Arnaud\Dropbox\Arena\node_modules\mongojs\node_modules\mongodb\l
ib\mongodb\collection\commands.js:54:5
    at C:\Users\Arnaud\Dropbox\Arena\node_modules\mongojs\node_modules\mongodb\l
ib\mongodb\db.js:1131:7
    at C:\Users\Arnaud\Dropbox\Arena\node_modules\mongojs\node_modules\mongodb\l
ib\mongodb\db.js:1847:9
    at Server.Base._callHandler (C:\Users\Arnaud\Dropbox\Arena\node_modules\mong
ojs\node_modules\mongodb\lib\mongodb\connection\base.js:445:41)
    at C:\Users\Arnaud\Dropbox\Arena\node_modules\mongojs\node_modules\mongodb\l
ib\mongodb\connection\server.js:478:18
    at MongoReply.parseBody (C:\Users\Arnaud\Dropbox\Arena\node_modules\mongojs\
node_modules\mongodb\lib\mongodb\responses\mongo_reply.js:68:5)
    at null.<anonymous> (C:\Users\Arnaud\Dropbox\Arena\node_modules\mongojs\node
_modules\mongodb\lib\mongodb\connection\server.js:436:20)
    at emit (events.js:95:17)
    at null.<anonymous> (C:\Users\Arnaud\Dropbox\Arena\node_modules\mongojs\node
_modules\mongodb\lib\mongodb\connection\connection_pool.js:201:13)
    at emit (events.js:98:17)

It happens not immediately after the line (but I am sure that this is the line that causes the error).

Do you know why please?