Web Part Error When populating sharepoint list item

Question

I have a people picker field in my custom web part and when the user fills the data and submits it i want this value to be populated in a share point list .following is the code i am using and this is giving me a error "The specified user not found" Can you please help me with this . Also i tested for other fields and those are populating without any error only this filed is giving me error..

SPWeb app = SPContext.Current.Web;
        SPList ListName = app.Lists["Communities Directory"];
        SPListItem ListItem = ListName.Items.Add();
        string[] UsersSeperated = pickAdministrators.CommaSeparatedAccounts.Split(',');
        SPFieldUserValueCollection UserCollection = new SPFieldUserValueCollection();
        foreach (string UserSeperated in UsersSeperated)
        {

            app.EnsureUser(UserSeperated);
            SPUser User = app.SiteUsers[UserSeperated];
            SPFieldUserValue UserName = new SPFieldUserValue(app, User.ID, User.LoginName);
            UserCollection.Add(UserName);
        }
        ListItem["Contact Person"] = UserCollection;

Answer 1

You can directly assign it as,

SPUser User = app.EnsureUser(UserSeperated);

In you above code, you are not using the value returned by the 'EnsureUser' method

Answer 2

Could you get the collection of the web using code below

  app.EnsureUser(UserSeperated);
  SPUser User = app.Site.RootWeb.SiteUsers[UserSeperated];

And then get specified user from this collection?