diff --git a/topics/memtopics.go b/topics/memtopics.go
index 3cddfa9..d316e88 100644
--- a/topics/memtopics.go
+++ b/topics/memtopics.go
@@ -125,8 +125,12 @@ func (this *memTopics) Retained(topic []byte, msgs *[]*message.PublishMessage) e
 }
 
 func (this *memTopics) Close() error {
+	this.smu.Lock()
 	this.sroot = nil
+	this.smu.Unlock()
+	this.rmu.Lock()
 	this.rroot = nil
+	this.rmu.Unlock()
 	return nil
 }