Error: throw new ERR_INVALID_CALLBACK. how to remove?

Question

Node.js

TypeError [ERR_INVALID_CALLBACK]: Callback must be a function

var fs = require("fs");
varutil = require('util');
varEOL = require('os').EOL;

function Log() {
// Nothing
}

module.exports = Log;

Log.prototype.setup = function(gameServer) {
if (!fs.existsSync('./logs')) {
// Make log folder
fs.mkdir('./logs');
}

switch (gameServer.config.serverLogLevel) {
case 2:
ip_log = fs.createWriteStream('./logs/ip.log', {flags : 'w'});

// Override
this.onConnect = function(ip) {
ip_log.write("["+this.formatTime()+"] Connect: " + ip + EOL);
};

this.onDisconnect = function(ip) {
ip_log.write("["+this.formatTime()+"] Disconnect: " + ip + EOL);
};
case 1:
console_log = fs.createWriteStream('./logs/console.log', {flags : 'w'});

console.log = function(d) { //
console_log.write(util.format(d) + EOL);
process.stdout.write(util.format(d) + EOL);
};

//
this.onCommand = function(command) {
console_log.write(">" + command + EOL);
case 0:
// Prevent crashes
process.on('uncaughtException', function(err) {
console.log(err.stack);
});
default:
break;
}
};

Log.prototype.onConnect = function(ip) {
// Nothing
};

Log.prototype.onDisconnect = function(ip) {
// Nothing
};

Log.prototype.onCommand = function(command) {
// Nothing
}

Log.prototype.formatTime = function() {
var date = new Date();

varhour = date.getHours();
hour = (hour < 10 ? "0" : "") + hour;

varmin = date.getMinutes();
min = (min < 10 ? "0" : "") + min;

return hour + ":" + min;
}