diff --git a/src/flask/app.py b/src/flask/app.py
index bd90a24b..3b6b38d8 100644
--- a/src/flask/app.py
+++ b/src/flask/app.py
@@ -1852,7 +1852,7 @@ class Flask(Scaffold):
 
         # extend existing headers with provided headers
         if headers:
-            rv.headers.update(headers)
+            rv.headers.update(headers)  # type: ignore[arg-type]
 
         return rv