android上的图像问题

我正在从url下载图像并设置为图像视图。但是有时位图返回空值? 为什么会这样?。。 我的密码是

Bitmap bm = null;
URL myFileUrl =null;          
    try 
    {
         myFileUrl= new URL(url);
    }
    catch (MalformedURLException e) 
    {
         // TODO Auto-generated catch block
         e.printStackTrace();
    }
    try 
    {
         HttpURLConnection conn= (HttpURLConnection)myFileUrl.openConnection();
         conn.setDoInput(true);
         conn.connect();
         int length = conn.getContentLength();
         InputStream is = conn.getInputStream();
         bm = BitmapFactory.decodeStream(is);

    } 
    catch (IOException e) 
    {
         // TODO Auto-generated catch block
         e.printStackTrace();
    }
    return bm;