diff --git a/cps/gdriveutils.py b/cps/gdriveutils.py
index f50a2f3f..08ead47d 100644
--- a/cps/gdriveutils.py
+++ b/cps/gdriveutils.py
@@ -147,7 +147,7 @@ engine = create_engine('sqlite:///{0}'.format(cli_param.gd_path), echo=False)
 Base = declarative_base()
 
 # Open session for database connection
-Session = sessionmaker()
+Session = sessionmaker(autoflush=False)
 Session.configure(bind=engine)
 session = scoped_session(Session)
 
diff --git a/cps/tasks/thumbnail.py b/cps/tasks/thumbnail.py
index e66da036..6d11fe97 100644
--- a/cps/tasks/thumbnail.py
+++ b/cps/tasks/thumbnail.py
@@ -138,7 +138,7 @@ class TaskGenerateCoverThumbnails(CalibreTask):
 
         # Replace outdated or missing thumbnails
         for thumbnail in book_cover_thumbnails:
-            if book.last_modified > thumbnail.generated_at:
+            if book.last_modified.replace(tzinfo=None) > thumbnail.generated_at:
                 generated += 1
                 self.update_book_cover_thumbnail(book, thumbnail)