diff --git a/comic/models.py b/comic/models.py
index 2bfd2ae..c9f64a3 100644
--- a/comic/models.py
+++ b/comic/models.py
@@ -64,7 +64,7 @@ class Directory(models.Model):
         return 'Directory'
 
     def generate_thumbnail(self) -> None:
-        book: ComicBook = ComicBook.objects.filter(directory=self).order_by('file_name').first()
+        book: ComicBook|None = ComicBook.objects.filter(directory=self).order_by('file_name').first()
         if not book:
             return
         if not book.thumbnail: