diff --git a/odoo/addons/base/models/assetsbundle.py b/odoo/addons/base/models/assetsbundle.py
index f1eaa9c7883..890b649ae16 100644
--- a/odoo/addons/base/models/assetsbundle.py
+++ b/odoo/addons/base/models/assetsbundle.py
@@ -538,7 +538,7 @@ class AssetsBundle(object):
 
     def is_css_preprocessed(self):
         preprocessed = True
-        attachments = None
+        old_attachments = self.env['ir.attachment']
         asset_types = [SassStylesheetAsset, ScssStylesheetAsset, LessStylesheetAsset]
         if self.user_direction == 'rtl':
             asset_types.append(StylesheetAsset)
@@ -549,6 +549,7 @@ class AssetsBundle(object):
             if assets:
                 assets_domain = self._get_assets_domain_for_already_processed_css(assets)
                 attachments = self.env['ir.attachment'].sudo().search(assets_domain)
+                old_attachments += attachments
                 for attachment in attachments:
                     asset = assets[attachment.url]
                     if asset.last_modified > attachment['__last_update']:
@@ -565,7 +566,7 @@ class AssetsBundle(object):
                 if outdated:
                     preprocessed = False
 
-        return preprocessed, attachments
+        return preprocessed, old_attachments
 
     def preprocess_css(self, debug=False, old_attachments=None):
         """