diff --git a/src/org/jgroups/protocols/UNICAST3.java b/src/org/jgroups/protocols/UNICAST3.java
index 2e87288d66..cf62131bce 100644
--- a/src/org/jgroups/protocols/UNICAST3.java
+++ b/src/org/jgroups/protocols/UNICAST3.java
@@ -1164,8 +1164,11 @@ protected void deliverBatch(MessageBatch batch) {
                 sb.append(" (" + batch.size()).append(" messages)");
                 log.trace(sb);
             }
-            if(stats)
-                avg_delivery_batch_size.add(batch.size());
+            if(stats) {
+                synchronized(avg_delivery_batch_size) { // can be accessed by different threads concurrently
+                    avg_delivery_batch_size.add(batch.size());
+                }
+            }
             up_prot.up(batch);
         }
         catch(Throwable t) {
diff --git a/src/org/jgroups/protocols/pbcast/NAKACK2.java b/src/org/jgroups/protocols/pbcast/NAKACK2.java
index a28f13872f..68aa893546 100644
--- a/src/org/jgroups/protocols/pbcast/NAKACK2.java
+++ b/src/org/jgroups/protocols/pbcast/NAKACK2.java
@@ -905,8 +905,12 @@ protected void removeAndDeliver(Table<Message> buf, Address sender, boolean loop
             }
             int size=batch.size();
             if(size > 0) {
-                if(stats)
-                    avg_batch_size.add(size);
+                if(stats) {
+                    synchronized(avg_batch_size) {
+                        // accessed by multiple threads concurrently
+                        avg_batch_size.add(size);
+                    }
+                }
                 deliverBatch(batch);
             }
         }