error when trying to use the completion handlers

gistfile1.txt

/************app.hjs******/

@import("Foundation");
@import("UIKit");

exports.backgroundDownloadTaskWithURL = function(identifier, url, callback) {
	if	(identifier != null) {
		var sessionConfig = NSURLSessionConfiguration.backgroundSessionConfiguration(identifier);
		var session = NSURLSession.sessionWithConfiguration(sessionConfig);
		var taskurl = NSURL.URLWithString(url);
		function completionHandler(data, response, error) {
			if (!error) {
				console.log("[DEBUG] Completed with data task with response :", response);
		    	var value = NSString.alloc().initWithData(data,NSUTF8StringEncoding);
				callback(value);
			} else {
				console.log("[WARN] Session completed with error :", error);
			}
		}
		var task = session.downloadTaskWithURL(taskurl, completionHandler);
		task.resume();
		
	} else {
		console.log("[WARN] Need to specify a proper identifier to create a URLSession.");
	}
}


/***********app.js***************/
var keyWindow = Ti.UI.createWindow({backgroundColor:"white"});
keyWindow.open();
var module = require("com.appcelerator.urlSession");

function callback(data) {
	Ti.API.info("Response Recieved");
	alert(data);
}
module.backgroundDownloadTaskWithURL("uniquevalue","http://api.openweathermap.org/data/2.5/weather?lat=37.389587&lon=-122.05037",callback);