diff --git a/v6d2ctx/pain.py b/v6d2ctx/pain.py
index c9fe58e..36b9d6e 100644
--- a/v6d2ctx/pain.py
+++ b/v6d2ctx/pain.py
@@ -61,7 +61,7 @@ class ABlockMonitor:
         self.delta = delta
         self.interval = interval
 
-    async def _monitor(self):
+    async def _monitor(self) -> None:
         while True:
             delta = self.delta
             t = time.time()