socket.io Can't get number of clients in room

I am making a game and I want to create 2-user rooms, when I try to get the number of users

io.sockets.emit('roomUsers', io.sockets.clients(openRoom).lenght);

node.js crashes

This is the log I get, I have no clue what does it mean:

/Users/ilya/game-app-1/node_modules/socket.io/lib/parser.js:75
      data = JSON.stringify(ev);
                  ^
TypeError: Converting circular structure to JSON
    at Object.stringify (native)
    at Object.exports.encodePacket (/Users/ilya/game-app-1/node_modules/socket.io/lib/parser.js:75:19)
    at SocketNamespace.packet (/Users/ilya/game-app-1/node_modules/socket.io/lib/namespace.js:154:23)
    at SocketNamespace.emit (/Users/ilya/game-app-1/node_modules/socket.io/lib/namespace.js:188:15)
    at Socket.<anonymous> (/Users/ilya/game-app-1/app.js:101:14)
    at Socket.EventEmitter.emit [as $emit] (events.js:96:17)
    at SocketNamespace.handlePacket (/Users/ilya/game-app-1/node_modules/socket.io/lib/namespace.js:335:22)
    at Manager.onClientMessage (/Users/ilya/game-app-1/node_modules/socket.io/lib/manager.js:488:38)
    at WebSocket.Transport.onMessage (/Users/ilya/game-app-1/node_modules/socket.io/lib/transport.js:387:20)
    at Parser.<anonymous> (/Users/ilya/game-app-1/node_modules/socket.io/lib/transports/websocket/hybi-16.js:39:10)