diff --git a/td/telegram/StickersManager.cpp b/td/telegram/StickersManager.cpp
index 158906eeb..37b1ba127 100644
--- a/td/telegram/StickersManager.cpp
+++ b/td/telegram/StickersManager.cpp
@@ -8158,6 +8158,19 @@ void StickersManager::on_new_stickers_uploaded(int64 random_id, Result<Unit> res
              pending_new_sticker_set->has_text_color_, std::move(input_stickers), pending_new_sticker_set->software_);
 }
 
+StickerFormat StickersManager::guess_sticker_set_format(const StickerSet *sticker_set) const {
+  auto format = StickerFormat::Unknown;
+  for (auto sticker_id : sticker_set->sticker_ids_) {
+    const auto *s = get_sticker(sticker_id);
+    if (format == StickerFormat::Unknown) {
+      format = s->format_;
+    } else if (format != s->format_) {
+      return StickerFormat::Unknown;
+    }
+  }
+  return format;
+}
+
 void StickersManager::add_sticker_to_set(UserId user_id, string short_name,
                                          td_api::object_ptr<td_api::inputSticker> &&sticker, Promise<Unit> &&promise) {
   TRY_RESULT_PROMISE(promise, input_user, td_->contacts_manager_->get_input_user(user_id));
@@ -8195,6 +8208,12 @@ void StickersManager::do_add_sticker_to_set(UserId user_id, string short_name,
   if (sticker_set == nullptr || !sticker_set->was_loaded_) {
     return promise.set_error(Status::Error(400, "Sticker set not found"));
   }
+  if (sticker != nullptr && sticker->format_ == nullptr) {
+    auto format = guess_sticker_set_format(sticker_set);
+    if (format != StickerFormat::Unknown) {
+      sticker->format_ = get_sticker_format_object(format);
+    }
+  }
 
   auto r_file_id = prepare_input_sticker(sticker.get(), sticker_set->sticker_type_);
   if (r_file_id.is_error()) {
@@ -8292,6 +8311,9 @@ void StickersManager::do_set_sticker_set_thumbnail(UserId user_id, string short_
     return promise.set_error(
         Status::Error(400, "The method can't be used to set thumbnail of custom emoji sticker sets"));
   }
+  if (format == StickerFormat::Unknown) {
+    format = guess_sticker_set_format(sticker_set);
+  }
 
   auto r_file_id = prepare_input_file(thumbnail, format, sticker_set->sticker_type_, true);
   if (r_file_id.is_error()) {
diff --git a/td/telegram/StickersManager.h b/td/telegram/StickersManager.h
index 663514a6c..a0282e882 100644
--- a/td/telegram/StickersManager.h
+++ b/td/telegram/StickersManager.h
@@ -812,6 +812,8 @@ class StickersManager final : public Actor {
 
   void on_added_sticker_uploaded(int64 random_id, Result<Unit> result);
 
+  StickerFormat guess_sticker_set_format(const StickerSet *sticker_set) const;
+
   void do_add_sticker_to_set(UserId user_id, string short_name, td_api::object_ptr<td_api::inputSticker> &&sticker,
                              Promise<Unit> &&promise);