diff --git a/src/flask/wrappers.py b/src/flask/wrappers.py
index e36a72cb..ca2ad017 100644
--- a/src/flask/wrappers.py
+++ b/src/flask/wrappers.py
@@ -151,7 +151,7 @@ class Response(ResponseBase):
         Added :attr:`max_cookie_size`.
     """
 
-    default_mimetype = "text/html"
+    default_mimetype: t.Optional[str] = "text/html"
 
     json_module = json