diff --git a/transport/internet/hysteria/dialer.go b/transport/internet/hysteria/dialer.go
index c652b40aa..1d7b8a95e 100644
--- a/transport/internet/hysteria/dialer.go
+++ b/transport/internet/hysteria/dialer.go
@@ -300,23 +300,21 @@ type clientManager struct {
 func (m *clientManager) clean() {
 	ticker := time.NewTicker(idleCleanupInterval)
 	for range ticker.C {
-		var forced []dialerConf
-
 		m.RLock()
+		entries := make(map[dialerConf]*client, len(m.m))
 		for k, c := range m.m {
+			entries[k] = c
+		}
+		m.RUnlock()
+		for k, c := range entries {
 			force := c.instance != nil && !c.instance.IsRunning()
 			c.clean(force)
 			if force {
-				forced = append(forced, k)
+				m.Lock()
+				delete(m.m, k)
+				m.Unlock()
 			}
 		}
-		m.RUnlock()
-
-		for i := range forced {
-			m.Lock()
-			delete(m.m, forced[i])
-			m.Unlock()
-		}
 	}
 }