diff --git a/httpx/_models.py b/httpx/_models.py
index 5af5c5d2..e1e45cf0 100644
--- a/httpx/_models.py
+++ b/httpx/_models.py
@@ -467,7 +467,7 @@ class Response:
         # the client will set `response.next_request`.
         self.next_request: typing.Optional[Request] = None
 
-        self.extensions = {} if extensions is None else extensions
+        self.extensions: ResponseExtensions = {} if extensions is None else extensions
         self.history = [] if history is None else list(history)
 
         self.is_closed = False