diff --git a/src/flask/sessions.py b/src/flask/sessions.py
index e63d1be5..0e403bbf 100644
--- a/src/flask/sessions.py
+++ b/src/flask/sessions.py
@@ -148,7 +148,7 @@ class SessionInterface:
     null_session_class = NullSession
 
     #: all sessions that are currently open
-    sessions = []
+    sessions: list[object] = []
 
     #: A flag that indicates if the session interface is pickle based.
     #: This can be used by Flask extensions to make a decision in regards