diff --git a/boards/abstracts/settingsmanager.py b/boards/abstracts/settingsmanager.py
--- a/boards/abstracts/settingsmanager.py
+++ b/boards/abstracts/settingsmanager.py
@@ -19,6 +19,8 @@ SETTING_LAST_NOTIFICATION_ID = 'last_not
 SETTING_IMAGE_VIEWER = 'image_viewer'
 SETTING_TRIPCODE = 'tripcode'
 
+FAV_THREAD_NO_UPDATES = -1
+
 DEFAULT_THEME = 'md'
 
 
@@ -124,8 +126,13 @@ class SettingsManager:
 
     def add_or_read_fav_thread(self, opening_post):
         threads = self.get_fav_threads()
-        threads[str(opening_post.id)] = opening_post.get_thread().get_replies()\
-            .last().id
+        thread = opening_post.get_thread()
+        # Don't check for new posts if the thread is archived already
+        if thread.is_archived():
+            last_id = FAV_THREAD_NO_UPDATES
+        else:
+            last_id = thread.get_replies().last().id
+        threads[str(opening_post.id)] = last_id
         self.set_setting(SETTING_FAVORITE_THREADS, threads)
 
     def del_fav_thread(self, opening_post):
diff --git a/boards/models/thread.py b/boards/models/thread.py
--- a/boards/models/thread.py
+++ b/boards/models/thread.py
@@ -116,7 +116,7 @@ class Thread(models.Model):
         Checks if the thread can be bumped by replying to it.
         """
 
-        return self.bumpable and not self.archived
+        return self.bumpable and not self.is_archived()
 
     def get_last_replies(self) -> QuerySet:
         """
@@ -233,3 +233,6 @@ class Thread(models.Model):
     def get_replies_newer(self, post_id):
         return self.get_replies().filter(id__gt=post_id)
 
+    def is_archived(self):
+        return self.archived
+
diff --git a/boards/views/api.py b/boards/views/api.py
--- a/boards/views/api.py
+++ b/boards/views/api.py
@@ -6,7 +6,8 @@ from django.db import transaction
 from django.http import HttpResponse
 from django.shortcuts import get_object_or_404
 from django.core import serializers
-from boards.abstracts.settingsmanager import get_settings_manager
+from boards.abstracts.settingsmanager import get_settings_manager,\
+        FAV_THREAD_NO_UPDATES
 
 from boards.forms import PostForm, PlainErrorList
 from boards.models import Post, Thread, Tag
@@ -256,8 +257,13 @@ def api_get_new_posts(request):
 
     for op in fav_thread_ops:
         last_read_post = fav_threads[str(op.id)]
-        new_posts = op.get_thread().get_replies_newer(last_read_post)
-        new_post_count = new_posts.count()
+
+        if last_read_post == FAV_THREAD_NO_UPDATES:
+            new_post_count = 0
+        else:
+            new_posts = op.get_thread().get_replies_newer(last_read_post)
+            new_post_count = new_posts.count()
+
         fav_thread_dict = dict()
         fav_thread_dict['id'] = op.id
         fav_thread_dict['new_post_count'] = new_post_count