Updating web configuration using SPWebConfigModificationType error

SPSecurity.RunWithElevatedPrivileges(delegate()
                    {
                     
                         //Get an instance of my local web application
                        Microsoft.SharePoint.Administration.SPWebApplication webApp = new SPSite("http://synpszd1241:3000/").WebApplication;

                       // SPSite site = new SPSite("http://synpszd1241:3000/");
                       // SPWeb spweb = site.OpenWeb();
                       // spweb.AllowUnsafeUpdates = true;
                       // site.AllowUnsafeUpdates = true;
                       //// spweb.ParentWeb.AllowUnsafeUpdates = true;
                       // spweb.Update();
                       // SPUtility.ValidateFormDigest();

                        Microsoft.SharePoint.Administration.SPWebConfigModification mod = new Microsoft.SharePoint.Administration.SPWebConfigModification("connectionStrings", "configuration");
                        mod.Owner = "SSO";
                        mod.Type = Microsoft.SharePoint.Administration.SPWebConfigModification.SPWebConfigModificationType.EnsureChildNode;
                        mod.Value = "<connectionStrings><clear/></connectionStrings>";

                        // Add my new web.config modification.
                        webApp.WebConfigModifications.Add(mod);
                        webApp.FormDigestSettings.Enabled = true;
                        webApp.Update();
                       
              %3