Invalid JSON TypeError

I am trying to call
nodejs_get_content_channel_users('jamio');

and in my console I receive the error:

getContentTokensUsers: Invalid JSON "{"channel":"jamio"}" [TypeError: Cannot read property 'sockets' of undefined]

I have console.log-ed my way through the code and it seems the culprit function is in server.js about line 480:

var getContentTokenChannelUsers = function (channel) {
  var users = {uids: [], authTokens: []};
  for (var sessionId in tokenChannels[channel].sockets) {
    if (io.sockets.sockets[sessionId].uid) {
      users.uids.push(io.sockets.sockets[sessionId].uid);
    }
    else {
      users.authTokens.push(io.sockets.sockets[sessionId].authToken);
    }
  }
  return users;
}

I really don't understand what this code is supposed to do other than loop through the users and emit their existance.