diff --git a/amqp_client_python/rabbitmq/async_channel.py b/amqp_client_python/rabbitmq/async_channel.py
index 37fe2b2..2adf069 100644
--- a/amqp_client_python/rabbitmq/async_channel.py
+++ b/amqp_client_python/rabbitmq/async_channel.py
@@ -473,6 +473,7 @@ async def rpc_subscribe(
 
         self.queue_bind(queue_name, exchange_name, routing_key)
         if not self.consumers[queue_name]:
+            assert self.ioloop is not None
             registered: Future[bool] = self.ioloop.create_future()
             self.consumers[queue_name] = True
             func = partial(self.on_message, queue_name)