Error saving documents to content tree

Hello Kentico

I am trying to save documents but i get an errors that is not consistent with the TreeNode information

TreeProvider Tree = new TreeProvider(MembershipContext.AuthenticatedUser);
TreeNode AuthorRoot = Tree.SelectNodes().Path("/authors").OnCurrentSite().Culture("en-us").FirstObject;
// this exists AuthorRoot, see image below
   TreeNode AuthorPage = TreeNode.New("CMS.Author", this.Tree);

        AuthorPage.DocumentName = authorName;
        AuthorPage.DocumentCulture = "en-us";
       // AuthorPage.DocumentPageTemplateID = 26076;

            // Creates the Author page
            AuthorRoot.Insert(AuthorPage);

I get these error

• [TreeNode.GetSiteName]: Site is not specified or specified site does not exist.
• [DocumentHelper.CheckParameters]: Parent node site not found.
• this line " parentAuthor.Insert(AuthorPage);"

I know there is a site associated with the tree node, but the save is not recognizing it

Any ideas?

Thanks