Cannot read property 'iAtlas' of undefined

/var/www/html/braincatalogue.org/js/atlasMakerServer.js:77
                                        if(Users[data.uid] &&
(Users[uid].iAtlas!=Users[data.uid].iAtlas))

 ^
TypeError: Cannot read property 'iAtlas' of undefined
    at WebSocket.<anonymous>
(/var/www/html/braincatalogue.org/js/atlasMakerServer.js:77:39)
    at WebSocket.emit (events.js:110:17)
    at Receiver.self._receiver.ontext
(/var/www/html/braincatalogue.org/js/node_modules/ws/lib/WebSocket.js:68
2:10)
    at Receiver.opcodes.1.finish
(/var/www/html/braincatalogue.org/js/node_modules/ws/lib/Receiver.js:391
:14)
    at Receiver.expectHandler
(/var/www/html/braincatalogue.org/js/node_modules/ws/lib/Receiver.js:372
:33)
    at Receiver.add
(/var/www/html/braincatalogue.org/js/node_modules/ws/lib/Receiver.js:87:
24)
    at Socket.firstHandler
(/var/www/html/braincatalogue.org/js/node_modules/ws/lib/WebSocket.js:66
3:22)
    at Socket.emit (events.js:107:17)
    at readableAddChunk (_stream_readable.js:161:16)
    at Socket.Readable.push (_stream_readable.js:128:10)
    at TCP.onread (net.js:514:20)