diff --git a/aio_pika/pool.py b/aio_pika/pool.py
index bba62b05..ecf968b4 100644
--- a/aio_pika/pool.py
+++ b/aio_pika/pool.py
@@ -142,7 +142,7 @@ async def __aexit__(
 
         await asyncio.ensure_future(self.close())
 
-    def __len__(self):
+    def __len__(self) -> int:
         return len(self.__item_set)