diff --git a/boards/forms.py b/boards/forms.py
--- a/boards/forms.py
+++ b/boards/forms.py
@@ -212,7 +212,7 @@ class PostForm(NeboardForm):
             for thread_id in threads_id_list:
                 try:
                     thread = Post.objects.get(id=int(thread_id))
-                    if not thread.is_opening():
+                    if not thread.is_opening() or thread.get_thread().archived:
                         raise ObjectDoesNotExist()
                     threads.append(thread)
                 except (ObjectDoesNotExist, ValueError):