diff --git a/substrate/client/network/src/service.rs b/substrate/client/network/src/service.rs
index 03b71b8c86..9bcde11e4b 100644
--- a/substrate/client/network/src/service.rs
+++ b/substrate/client/network/src/service.rs
@@ -1549,7 +1549,9 @@ impl<B: BlockT + 'static, H: ExHashT> Future for NetworkWorker<B, H> {
 					}
 					{
 						let mut peers_notifications_sinks = this.peers_notifications_sinks.lock();
-						peers_notifications_sinks.insert((remote.clone(), protocol.clone()), notifications_sink);
+						let _previous_value = peers_notifications_sinks
+							.insert((remote.clone(), protocol.clone()), notifications_sink);
+						debug_assert!(_previous_value.is_none());
 					}
 					this.event_streams.send(Event::NotificationStreamOpened {
 						remote,
@@ -1569,6 +1571,7 @@ impl<B: BlockT + 'static, H: ExHashT> Future for NetworkWorker<B, H> {
 							target: "sub-libp2p",
 							"NotificationStreamReplaced for non-existing substream"
 						);
+						debug_assert!(false);
 					}
 
 					// TODO: Notifications might have been lost as a result of the previous
@@ -1603,7 +1606,9 @@ impl<B: BlockT + 'static, H: ExHashT> Future for NetworkWorker<B, H> {
 					});
 					{
 						let mut peers_notifications_sinks = this.peers_notifications_sinks.lock();
-						peers_notifications_sinks.remove(&(remote.clone(), protocol));
+						let _previous_value = peers_notifications_sinks
+							.remove(&(remote.clone(), protocol));
+						debug_assert!(_previous_value.is_some());
 					}
 				},
 				Poll::Ready(SwarmEvent::Behaviour(BehaviourOut::NotificationsReceived { remote, messages })) => {