diff --git a/lcl/include/customnotebook.inc b/lcl/include/customnotebook.inc
index 0e6a0e6261..42e519f261 100644
--- a/lcl/include/customnotebook.inc
+++ b/lcl/include/customnotebook.inc
@@ -449,6 +449,12 @@ begin
   Result := AIndex;
   if csDesigning in ComponentState then
     exit; //all pages are visible
+
+  // it is possible to show pages without visible tabs, but then, no tab index
+  // can be sendet back, issue #21723
+  if not Page[AIndex].TabVisible then
+    exit(-1);
+
   for i := 0 to AIndex - 1 do begin
     if not Page[i].TabVisible then
       dec(Result); //exclude invisible page
diff --git a/lcl/interfaces/gtk2/gtk2pagecontrol.inc b/lcl/interfaces/gtk2/gtk2pagecontrol.inc
index 42e605247f..626f83666a 100644
--- a/lcl/interfaces/gtk2/gtk2pagecontrol.inc
+++ b/lcl/interfaces/gtk2/gtk2pagecontrol.inc
@@ -553,6 +553,9 @@ begin
     Exit;
   if (AIndex < 0) or (AIndex > ATabControl.PageCount - 1) then
     exit;
+  ANewIndex:=ATabControl.PageToTabIndex(AIndex);
+  if (ANewIndex < 0) then
+    exit;
   GtkNotebook := {%H-}PGtkNoteBook(ATabControl.Handle);
   if gtk_notebook_get_current_page(GtkNotebook) <> AIndex then
   begin