diff --git a/waku/v2/rest/relay.go b/waku/v2/rest/relay.go
index 24e85574..9c73796b 100644
--- a/waku/v2/rest/relay.go
+++ b/waku/v2/rest/relay.go
@@ -71,11 +71,13 @@ func (r *RelayService) Start(ctx context.Context) {
 	ctx, cancel := context.WithCancel(ctx)
 	r.cancel = cancel
 
+	r.messagesMutex.Lock()
 	// Node may already be subscribed to some topics when Relay API handlers are installed. Let's add these
 	for _, topic := range r.node.Relay().Topics() {
 		r.log.Info("adding topic handler for existing subscription", zap.String("topic", topic))
 		r.messages[topic] = []*pb.WakuMessage{}
 	}
+	r.messagesMutex.Unlock()
 
 	r.runner.Start(ctx)
 }
diff --git a/waku/v2/rpc/relay.go b/waku/v2/rpc/relay.go
index 46e4c527..92619ffe 100644
--- a/waku/v2/rpc/relay.go
+++ b/waku/v2/rpc/relay.go
@@ -67,11 +67,13 @@ func (r *RelayService) addEnvelope(envelope *protocol.Envelope) {
 }
 
 func (r *RelayService) Start() {
+	r.messagesMutex.Lock()
 	// Node may already be subscribed to some topics when Relay API handlers are installed. Let's add these
 	for _, topic := range r.node.Relay().Topics() {
 		r.log.Info("adding topic handler for existing subscription", zap.String("topic", topic))
 		r.messages[topic] = make([]*pb.WakuMessage, 0)
 	}
+	r.messagesMutex.Unlock()
 
 	r.runner.Start()
 }