diff --git a/waku/v2/rest/relay.go b/waku/v2/rest/relay.go
index b5ffc8a7..5f6de6d6 100644
--- a/waku/v2/rest/relay.go
+++ b/waku/v2/rest/relay.go
@@ -128,8 +128,9 @@ func (d *RelayService) postV1Subscriptions(w http.ResponseWriter, r *http.Reques
 			d.log.Error("subscribing to topic", zap.String("topic", strings.Replace(topicToSubscribe, "\n", "", -1)), zap.Error(err))
 		} else {
 			d.node.Broadcaster().Unregister(&topicToSubscribe, sub.C)
-
+			d.messagesMutex.Lock()
 			d.messages[topic] = []*pb.WakuMessage{}
+			d.messagesMutex.Unlock()
 		}
 	}
 
diff --git a/waku/v2/rpc/relay.go b/waku/v2/rpc/relay.go
index 6de142e3..6bb3b982 100644
--- a/waku/v2/rpc/relay.go
+++ b/waku/v2/rpc/relay.go
@@ -116,7 +116,9 @@ func (r *RelayService) PostV1Subscription(req *http.Request, args *TopicsArgs, r
 			r.log.Error("subscribing to topic", zap.String("topic", topic), zap.Error(err))
 			return err
 		}
+		r.messagesMutex.Lock()
 		r.messages[topic] = make([]*pb.WakuMessage, 0)
+		r.messagesMutex.Unlock()
 	}
 
 	*reply = true