Adding list items to a folder with in the list, SharePoint add item to custom list results in “Invalid URL value. A URL field contains invalid data. Please check the value and try again”

protected void submit_click(object sender, EventArgs e)

        {

            using (SPSite site = SPContext.Current.Site)

            {

                SPWeb web = site.OpenWeb();

                web.AllowUnsafeUpdates = true;

                bool isFolderExist = false;

                DateTime dt = DateTime.Now;

                SPListItem folderColl_item;

                SPListItemCollection folderColl;

                SPList list = web.Lists["ListName"];

                folderColl = list.Folders;

                foreach (SPListItem splistitem in list.Folders)

                {

                    SPFolder folder = splistitem.Folder;

                    if (folder.Name.Equals(dt.Year.ToString()))

                    {

                        isFolderExist = true;

                    }

                }

                if (!isFolderExist)

                {

                    folderColl_item = list.Items.Add(list.RootFolder.ServerRelativeUrl, SPFileSystemObjectType.Folder, dt.Year.ToString());

                    folderColl_item.Update();

                    list.Update();

                }

                folderColl = list.Folders;

                foreach (SPListItem splistitem in list.Folders)

                {

                    SPFolder folder = splistitem.Folder;

                    if (folder.Name.Equals(dt.Year.ToString()))

                    {

                        SPListItemCollection itemColl = list.Items;

                        SPListItem item = itemColl.Add(folder.ServerRelativeUrl, SPFileSystemObjectType.File, null);

                        item["test"] = "test";

                        item.Update();

                        list.Update();

                    }

                }

                web.AllowUnsafeUpdates = false;

            }

        }