diff --git a/broker/src/main/java/io/moquette/imhandler/GroupHandler.java b/broker/src/main/java/io/moquette/imhandler/GroupHandler.java
index ff0e02f8..62670b33 100644
--- a/broker/src/main/java/io/moquette/imhandler/GroupHandler.java
+++ b/broker/src/main/java/io/moquette/imhandler/GroupHandler.java
@@ -19,6 +19,8 @@ abstract public class GroupHandler<T> extends IMHandler<T> {
     protected void sendGroupNotification(String fromUser, String targetId, List<Integer> lines, WFCMessage.MessageContent content) {
         if (lines == null) {
             lines = new ArrayList<>();
+        } else {
+            lines = new ArrayList<>(lines);
         }
 
         if (lines.isEmpty()) {