diff --git a/hikka/tl_cache.py b/hikka/tl_cache.py
index 0b5d34b..f92f97a 100644
--- a/hikka/tl_cache.py
+++ b/hikka/tl_cache.py
@@ -253,7 +253,7 @@ def install_perms_caching(client: TelegramClient):
     async def cleaner(client: TelegramClient):
         while True:
             for chat, chat_data in client._hikka_perms_cache.copy().items():
-                for user, user_data in chat_data.items():
+                for user, user_data in chat_data.items().copy():
                     if user_data.expired():
                         del client._hikka_perms_cache[chat][user]
                         logger.debug(f"Cleaned outdated perms cache {chat=} {user=}")