diff --git a/backend/open_webui/models/users.py b/backend/open_webui/models/users.py
index 60addaa..9d0cb93 100644
--- a/backend/open_webui/models/users.py
+++ b/backend/open_webui/models/users.py
@@ -346,7 +346,7 @@ class UsersTable:
             users = db.query(User).filter(User.id.in_(user_ids)).all()
             return [user.id for user in users]
 
-    def generate_letter_image(letter, image_size=(200, 200), font_size=100, font_path=None):
+    def generate_letter_image(letter: str, image_size=(200, 200), font_size=100, font_path=None):
         """
         Generuje obraz z podaną literą i losowym kolorem tła, zwracając go w formacie base64.