diff --git a/staging/src/k8s.io/apiserver/pkg/storage/cacher/cacher.go b/staging/src/k8s.io/apiserver/pkg/storage/cacher/cacher.go
index e366906af4e..925ef0d2c69 100644
--- a/staging/src/k8s.io/apiserver/pkg/storage/cacher/cacher.go
+++ b/staging/src/k8s.io/apiserver/pkg/storage/cacher/cacher.go
@@ -163,7 +163,6 @@ func (i *indexedWatchers) terminateAll(objectType reflect.Type, done func(*cache
 // As we don't need a high precision here, we keep all watchers timeout within a
 // second in a bucket, and pop up them once at the timeout. To be more specific,
 // if you set fire time at X, you can get the bookmark within (X-1,X+1) period.
-// This is NOT thread-safe.
 type watcherBookmarkTimeBuckets struct {
 	lock            sync.Mutex
 	watchersBuckets map[int64][]*cacheWatcher
diff --git a/staging/src/k8s.io/apiserver/pkg/storage/cacher/cacher_whitebox_test.go b/staging/src/k8s.io/apiserver/pkg/storage/cacher/cacher_whitebox_test.go
index ff807584029..fa2c921920b 100644
--- a/staging/src/k8s.io/apiserver/pkg/storage/cacher/cacher_whitebox_test.go
+++ b/staging/src/k8s.io/apiserver/pkg/storage/cacher/cacher_whitebox_test.go
@@ -647,7 +647,12 @@ func TestCacherNoLeakWithMultipleWatchers(t *testing.T) {
 	cacher.bookmarkWatchers.lock.Lock()
 	defer cacher.bookmarkWatchers.lock.Unlock()
 	if len(cacher.bookmarkWatchers.watchersBuckets) != 0 {
-		t.Errorf("unexpected bookmark watchers %v", len(cacher.bookmarkWatchers.watchersBuckets))
+		numWatchers := 0
+		for bucketID, v := range cacher.bookmarkWatchers.watchersBuckets {
+			numWatchers += len(v)
+			t.Errorf("there are %v watchers at bucket Id %v with start Id %v", len(v), bucketID, cacher.bookmarkWatchers.startBucketID)
+		}
+		t.Errorf("unexpected bookmark watchers %v", numWatchers)
 	}
 }