socket.io warn - unknown transport: "flashsocket"

Default setting, "flashsocket" is not enabled.
Describe following souce to any place at souce code.

var io = require('socket.io').listen(8888);
io.set('transports', [
    'websocket'
  , 'flashsocket'
  , 'htmlfile'
  , 'xhr-polling'
  , 'jsonp-polling'
  ]);