diff --git a/src/org/jgroups/util/AckTable.java b/src/org/jgroups/util/AckTable.java
index 51959823b3..8a5eeec880 100644
--- a/src/org/jgroups/util/AckTable.java
+++ b/src/org/jgroups/util/AckTable.java
@@ -1,10 +1,14 @@
 package org.jgroups.util;
 
 import org.jgroups.Address;
+import org.jgroups.annotations.GuardedBy;
 
+import java.util.HashMap;
 import java.util.List;
 import java.util.Map;
 import java.util.Optional;
+import java.util.concurrent.locks.Lock;
+import java.util.concurrent.locks.ReentrantLock;
 import java.util.stream.Collectors;
 
 /**
@@ -13,39 +17,83 @@
  * @since  5.4
  */
 public class AckTable {
-    protected final Map<Address,Long> acks=Util.createConcurrentMap();
-    protected long                    min; // the current minimum, recomputed on ack() and view change
+    protected final Map<Address,Long> acks=new HashMap<>();
+    protected final Lock              lock=new ReentrantLock();
+    protected volatile long           min; // the current minimum, recomputed on ack() and view change
 
-    public long min() {return min;}
+    public long min() {
+        lock.lock();
+        try {
+            return min;
+        }
+        finally {
+            lock.unlock();
+        }
+    }
 
     /** Adds an ACK from a sender to the map. Returns the new minimum */
     public long ack(Address sender, long seqno) {
-        Long existing=acks.get(sender);
-        if(existing != null && existing >= seqno)
-            return min;
-        acks.put(sender, seqno);
-        return min=computeMin();
+        // System.out.printf("-- [%d] ACK(%s,%d)\n", Thread.currentThread().getId(), sender, seqno);
+        lock.lock();
+        try {
+            Long existing=acks.get(sender);
+            if(existing != null && existing >= seqno)
+                return min;
+            acks.put(sender, seqno);
+            return min=computeMin();
+        }
+        finally {
+            lock.unlock();
+        }
     }
 
     /** Removes left members from and adds new members to the map */
     public AckTable adjust(List<Address> mbrs) {
         if(mbrs == null)
             return this;
-        acks.keySet().retainAll(mbrs);
-        for(Address mbr: mbrs)
-            acks.putIfAbsent(mbr, 0L);
-        min=computeMin();
-        return this;
+        lock.lock();
+        try {
+            acks.keySet().retainAll(mbrs);
+            for(Address mbr: mbrs)
+                acks.putIfAbsent(mbr, 0L);
+            min=computeMin();
+            return this;
+        }
+        finally {
+            lock.unlock();
+        }
     }
 
-    public int size() {return acks.size();}
+    public AckTable clear() {
+        lock.lock();
+        try {
+            acks.clear();
+            min=computeMin();
+            return this;
+        }
+        finally {
+            lock.unlock();
+        }
+    }
+
+    public int size() {
+        lock.lock();
+        try {
+            return acks.size();
+        }
+        finally {
+            lock.unlock();
+        }
+    }
 
+    // no need for the lock - approximation, may be incorrect, that's ok
     @Override public String toString() {
-        String tmp= acks.entrySet().stream().map(e -> String.format("%s: %d", e.getKey(), e.getValue()))
+        String tmp=acks.entrySet().stream().map(e -> String.format("%s: %d", e.getKey(), e.getValue()))
           .collect(Collectors.joining("\n"));
         return tmp.isEmpty()? "min: " + min : tmp + "\nmin: " + min;
     }
 
+    @GuardedBy("lock")
     protected long computeMin() {
         Optional<Long> m=acks.values().stream().min(Long::compareTo);
         return m.orElse(min);
diff --git a/tests/junit-functional/org/jgroups/tests/AckTableTest.java b/tests/junit-functional/org/jgroups/tests/AckTableTest.java
index 1b6d56668f..5064d2608b 100644
--- a/tests/junit-functional/org/jgroups/tests/AckTableTest.java
+++ b/tests/junit-functional/org/jgroups/tests/AckTableTest.java
@@ -64,5 +64,9 @@ public void testAck() {
         table.adjust(List.of(a,b,c,d));
         assert table.size() == 4;
         assert table.min() == 0;
+
+        table.clear();
+        assert table.size() == 0;
+        assert table.min() == 0;
     }
 }