diff --git a/src/org/jgroups/protocols/UNICAST3.java b/src/org/jgroups/protocols/UNICAST3.java
index ca45458228..74e5b0d12a 100644
--- a/src/org/jgroups/protocols/UNICAST3.java
+++ b/src/org/jgroups/protocols/UNICAST3.java
@@ -139,7 +139,7 @@ public class UNICAST3 extends Protocol implements AgeOutCache.Handler<Address> {
 
     /** Used by the retransmit task to keep the last retransmitted seqno per member (applicable only
      * for received messages (ReceiverEntry)): https://issues.redhat.com/browse/JGRP-1539 */
-    protected final Map<Address,Long>      xmit_task_map=new HashMap<>();
+    protected final Map<Address,Long>      xmit_task_map=new ConcurrentHashMap<>();
 
     /** RetransmitTask running every xmit_interval ms */
     protected Future<?>                    xmit_task;