diff --git a/11-testing/87-time-api/listing1/main.go b/11-testing/87-time-api/listing1/main.go
index 5b483ec..8d9ed68 100644
--- a/11-testing/87-time-api/listing1/main.go
+++ b/11-testing/87-time-api/listing1/main.go
@@ -16,8 +16,8 @@ type Event struct {
 }
 
 func (c *Cache) TrimOlderThan(since time.Duration) {
-	c.mu.RLock()
-	defer c.mu.RUnlock()
+	c.mu.Lock()
+	defer c.mu.Unlock()
 
 	t := time.Now().Add(-since)
 	for i := 0; i < len(c.events); i++ {
diff --git a/11-testing/87-time-api/listing2/main.go b/11-testing/87-time-api/listing2/main.go
index 6d0d944..41c8285 100644
--- a/11-testing/87-time-api/listing2/main.go
+++ b/11-testing/87-time-api/listing2/main.go
@@ -26,8 +26,8 @@ type Event struct {
 }
 
 func (c *Cache) TrimOlderThan(since time.Duration) {
-	c.mu.RLock()
-	defer c.mu.RUnlock()
+	c.mu.Lock()
+	defer c.mu.Unlock()
 
 	t := time.Now().Add(-since)
 	for i := 0; i < len(c.events); i++ {